Q.You are a business traveler on the road, and suddenly you have a complicated, last-minute change in itinerary. It means a switch in airline, hotel and car-rental reservations. And to make things worse, the travel agency that booked your trip has closed for the day.

It's a predicament that is far from unusual, say the operators of a relatively new type of travel service -- the 24-hour-a-day, 365-day-a-year travel agency. There's a small group of them, probably less than a dozen, operating nationwide to provide off-hours help, primarily to the business traveler within the United States.

Faced with an altered itinerary, you could spend the evening making new arrangements yourself, which might involve first finding out who flies where you want to go. Or you could call one of these all-hours travel firms and let them do all the work.

This kind of service is available to travelers who book their trips through a travel agency that subscribes to one of the all-hours operations. The travel agency gives its clients individual code numbers and a no-charge 800 number to phone should an emergency arise.

When you phone, the after-hours staff can call up your travel records on their computer, make new hotel and car-rental reservations and have the adjusted tickets waiting for you the next morning at the airline counter. Your travel agent gets any commissions.

Typically, a travel agency offers this service to its customers for no extra charge. Some make it available to both vacation and business travelers, and others restrict it to their best customers, such as large corporations.

A 24-hour operation may sell its services for a monthly rate or charge by the number of phone calls received, which is one reason a travel agency might want to limit the use.

Roberta Terzo, director of Travel Helpline of Great Neck, N.Y., recalls a situation in which a Cleveland caller was given orders by his boss on Sunday to fly the next day to South Africa. While he was busy packing and getting his briefcase in order, the Helpline staff took care of plane and hotel reservations and had the tickets waiting.

Another of these travel services, Teletix, is based in Washington. It originated last year as something extra the Georgetown Ticket Center, a travel agency, could offer its clients. The service turned out to be "very, very popular," says agency president Bruce Bishins. Now about 200 other travel agencies subscribe to Teletix.

"We're a travel agent's travel agency," says Steven Trooboff, president of Corporate Services International of Cambridge, Mass. His firm offers the Travelers Emergency Service System (TESS), providing a 24-hour operation subscribed to by 1,200 travel agents across the country.

TESS, now 5 years old, was an early entry into the off-hours service, aided, says Trooboff, by the development of computer reservation networks and the availability of a national toll-free 800 phone system. He estimates there are about 10 companies nationwide providing a similar service for about 4,000 of the country's 25,000 travel agencies.

When his firm began, it averaged about 2,000 calls from travelers a month. The figure has grown to 25,000 a month. About 60 percent of them are requests for changes in reservations, and many of the rest involve such problems as an overbooked hotel or no record of a reservation when the customer shows up at a car-rental desk.

Perhaps the busiest times for his service, says Trooboff, are when weather, particularly a major snowstorm, disrupts air traffic throughout the East. Then the phones don't stop ringing as travelers try to reschedule hotel rooms and get aboard the first available flight.

Trooboff expects that increasing numbers of travel agencies will subscribe to a 24-hour service, and Teletix's Bishins agrees: "I suspect that as clients become more and more aware of the service," he says, "a travel agency will have to offer it." And Travel Helpline's Terzo adds: "Whether you are a frequent traveler or not, for the same price you can find an agency that has this service."

Some large travel agency chains, such as Ask Mr. Foster of Van Nuys, Calif., which has 304 branch offices, provide their own 24-hour operation staffed, as senior vice president Diane Steenman puts it, "by our own employes." The service is called Executive Hotline.

The industry is mixed on the question of whether vacation travelers can benefit from similar after-hours help. Ask Mr. Foster's hotline is available only to business travelers, says Steenman, because problems developing from typical vacation trips, such as cruises and tours, aren't readily remedied at night.

However, the Travel Helpline, which has 450 subscribing travel agencies, "is designed to handle commercial as well as vacation travel," says Terzo. "The vacationer needs us because they are the inexperienced traveler. If something goes wrong, they can be taken advantage of."

Only a small percentage of incoming calls are real emergencies, say the operators of these after-hours services. Many travelers simply like the convenience of dealing with a travel agent after regular business hours. Early risers call at 5 a.m. to get a head start on their day.

* ROOM SERVICE: Convention-goers know the problem. When everyone who is due at the same 8 a.m. meeting phones hotel room service for breakfast at 7:30, sometimes it doesn't show up on time.

That happened to a Washington lobbyist on business in Atlanta recently. By the time her meeting began, the promised breakfast had not arrived, and she canceled it. When she returned to her room later in the morning, however, a fruit basket awaited with a note of apology from the management.

"We do that all the time if there's a delay," says Joe Morgan, assistant room service manager of downtown Atlanta's Colony Square Hotel.

Morgan points out that an overburdened kitchen is not always to blame. Frequently, he says, so many guests are leaving their rooms at once that the room-service staff has difficulty catching an elevator to deliver the breakfast tray.

* EXECUTIVE TRAVEL: A boxed invitation -- about the size of a three-record album -- was sent recently to 200 "chief executives" in Washington and Baltimore, inviting them on a deluxe tour of England that includes a Concorde flight out of Baltimore/Washington airport and a cruise back to Baltimore aboard the Queen Elizabeth 2.

The tour, organized by Travel Destinations of Owings Mills, Md., highlights the luxury liner's inaugural departure from the Port of Baltimore. The tour, dubbed "The Queen's Concordance," is scheduled for April 26 to May 5, and the price ranges from $4,250 to $6,090 per person, depending on the location of the cabin aboard ship.

For that price, the executives (or anyone who has the price of a ticket) get private limousine transportation to the airport; the 3-hour-and-45-minute Concorde flight to London; three nights at the Churchill Hotel in London; an afternoon's outing to Leeds Castle in Kent aboard cars of the famed Orient Express; and a six-night cruise home.

Since this is the maiden voyage of the QE-2 to Baltimore, it will be greeted by a great deal of hoopla, including a fireboat display and an accompanying armada of smaller craft.

For information: (202) 393-6653.