If Hollywood were handling it, it might be called "The Day The Government Stood Still." But even if a confrontation between President Reagan and Congess leaves the federal government without money as of midnight tonight, government agencies will still be open Monday.

However, it decidedly would not be business-as-usual for many bureaucrats and the people they serve.

In each agency and department, from the bureaucratic giants like the Health and Human Services Department (157,000 employes) to the Administrative Conference of the United States (22 employes) supervisors are faced with dozens of decisions on what work will be done and what won't, which checks are written and which aren't and, if a spending halt continues beyond a day or two, which employes get furloughed and which can continue to work. [Story on Page B2]

Many government functions would continue without a hitch, either because they don't involve annual congressional appropriations or because they are essential to the protection of life and property and have been exempted from the shutdown by the Office of Management and Budget.

Mail will be delivered and air traffic controllers will keep planes flying. The nation's borders will stay open and will be patrolled and the Treasury Department will continue to borrow money, make interest payments, collect taxes and do whatever is necessary to keep the banking community functioning. Prison guards will stay on the job as will the military.

Social Security checks would keep going out, although that point may be moot since they were sent out at the beginning of the month and few expect the government will still be shut down on Dec. 1. But checks under the Supplemental Security Income Program, which gets an annual appropriation, would probably stop.

So would a lot of other things. Most regulatory agencies, like the Federal Communications Commission and the Federal Trade Commission would have little or no authority to write rule or enforce them. The FCC probably couldn't rule on license applications and the FTC -- which went through this exercise in May, 1980, when its funding was cut off for a day -- couldn't go ahead with its antitrust cases.

And, at least theoretically, thousands of checks to everyone from military pensioners to scientists working on research grants to highway builders to schools for the handicapped couldn't be written.

The question of government grants, however, is tricky because some grant programs are part of the budget, some are financed by independent trust funds and some are a combination. Checks from independent funds, like Social Security, should continue regardless; checks for families receiving aid to dependent children or checks going to state and local governments for school lunches might well stop.

If the government were shut down for a prolonged period, many of these questions could end up in the courts. However, the judiciary's money would be in the same limbo as everyone else's, so there might not be any federal courts in session.

"Frankly, the matter hasn't been given much thought yet," said Edward Garabedian, a spokesman for the Administrative Office of the U.S. Courts. "We'll wait until Congress acts. Or until the executive branch gives us an indication of what they'll do with the rest of the government."

Dozens of attorneys in the executive branch were trying to figure that out yesterday after an OMB meeting on the subject. The legal issues had already been interpreted by former attorney general Benjamin R. Civiletti in opinions written in April, 1980, and January, 1981.

The first, drawn up at the time of the FTC shutdown, said the only work agency employes could legally perform was packing up files and closing down their agencies. The second said the president has broad authority to approve funding for emergency operations, an interpretation of executive authority that constitutional scholars might dispute.

In the Treasury and Justice departments, memos went out telling employes that they might not be paid for work done after Monday, and asking that they make lists of pending cases and assignments so a skeleton crew could keep track of court dates and meetings to be canceled and other business that had to be put off.

"We're operating on the assumption that there will be a continuing resolution or an appropriation," said Michael Horowitz, counsel to OMB Director David A. Stockman. "The activity involved is a suspension of operations," not a closing of business, he added.

In most cases, OMB is leaving it up to top agency officials to find their answers. OMB did rule that scientific experiments involving animals or critical measurements should not stop. Feeding experimental rats, in effect, constitutes protecting government property.

On Capitol Hill, employes had no immediate worries about shutting down. Funds for Congress and the salaries of its 40,000 employes were authorized through Sept. 30, 1982, by a continuing resolution approved last month.