Marjorie Tyler, a Prince George's County window, is suffering through a mother's worst fear: Her son is in peril, his fate is uncertain, and there is little, if anything, that she can do about it.

Fourteen weeks ago, while flying a single-engine plane on a business trip to South Africa, Geoffrey H. Tyler, 31, inexplicably landed on a road in Angola. Nothing has been heard from him since. According to State Department sources, the Marxist government of that country has put him in prison.

Marge Tyler, a corporate secretary for Giant Food, Inc. in Landover, has continued to work, but says she "goes into hysterics" about once a week.

"I cry, I wonder if he has malaria, if they're feeding him, if he'll ever get out," she said. "He has done nothing wrong. Why can't anybody do anything about my son?"

One of the problems is that the United States has no diplomatic relations with Angola, a country twice the size of Texas located on the west coast of southern Africa. Government officials there, in spite of pleas from the State Department, have not even acknowedged that they have Tyler. But Italian diplomats, acting as intermediaries, have confirmed it.

""We know that he is in Sao Paulo Prison in Luanda, that he is in good health, and that American prisoners are generally afforded better treatment than their Angolan counterparts," said Jim Overly, head of the Angola desk at the State Department.

Overly and senior department officials, on a trip through Africa last month, asked to see Tyler and requested his release. Angolan foreign ministry officials simply took note of the requests and gave no response, State Department officials said. The Italians have asked to see him and have been similarly treated, they said.

Last week a magazine in Mozambique, purportedly quoting Angolan sources, said that Angola is planning to charge Tyler with spying for the CIA and passing information to pro-Western UNITA guerrillas trying to take over the country.

State Department sources say the CIA has told them Tyler had no connection with that agency. These sources say that the magazine, "Tempo," is an organ of the ruling Marxist party of Mozambique and the accuracy of its reports generally are in question.

"I don't see how they could have a case against him," Overly said.

Rep. Robert Dornan (R-Calif.), a member of the House Africa subcommittee who has been contacted by the Tyler family, said the CIA has assured him that Tyler "has absolutely no connection" with that agency.

"He should have been released within 48 hours after he landed," Dornan said. "His name should be a household word by now [as a prisoner], and nobody even knows about it."

Marge Tyler reacted to the magazine report with more dismay.

"There is not a bit of truth in that," she said."He is strictly a 'flyboy,' a downed pilot. I don't know why they are holding him."

Two other Americans are in the same prison as Tyler: Gary Acker of Sacramento, Calif., and Gustavo Grillo of Jersey City, N.J. Both are serving long sentences after their 1976 conviction as mercenaries in Angola's civil war. Another American, Daniel Gearhart of Kensington, Md., was executed after his conviction on similar charges.

Marge Tyler, 53, said her son is a 1967 graduate of Duval High School who has been living in Lakeland, Fla. and working as a civilian pilot for Globe Aero Ltd., Inc., a firm that ferries small aircraft to purchasers abroad.

Tyler was enroute to Cape Town in a single-engine Piper Cherokee Arrow when he disappeared on Feb. 4, according to Globe Aero's president, Phil Waldman.

He had left the Ivory Coast capital of Abidjan and was to refuel in Windhoek, Namibia, when he went off course. Waldman said the flight plan should have kept him about 100 miles off the Angolan coast. His last radio transmission, picked up by a nearby commercial 707 flight, was, "I have Walvis Bay in sight," according to Waldman and accounts in South African newspapers.

Air controllers at Windhoek immediately realized that this report was in error, because Walvis Bay was covered from view by low clouds, according to the Windhoek Observer. The South African Air Force searched for two days before sources whom the State Department refuses to identify reported that Tyler had landed in south-central Angola.

Tyler should have had ample fuel, since the planes he flies are equipped with extra tanks for the ferry trips, and there was no indication of instrument or weather problems, Waldman said. Tyler had ferried planes to South Africa about 25 times and should have been familiar with the terrain, Waldman added.

"It's almost impossible to say what happened to the man without talking to him," Waldman said.

One of Globe Aero's pilots, on a similar run, had oil pressure problems last year and made a forced landing in Angola. He was kept for six months without charges, then abruptly released. That time, however, Belgian diplomats had been able to see the prisoner and he had been allowed to send and receive mail, Waldman said.

Because of these previous problems, Globe Aero filed a flight plan with the Angolans in advance of this latest trip, Waldman said.

When she found out that her son was down in Angola, Tyler said, she was dumbstruck.

"I thought, 'Oh my God, not Angola, don't tell me he's there.'" she said.

She and other relatives have contacted numerous members of Congress and have written to Secretary of State Alexander Haig.

"The State Department seems to be doing all it can; it's tremendously difficult when you have no one on the ground there," said Marvin (Bud) Moss, administrative assistant to Sen. Paul S. Sarbanes (D-Md.).

Dornam said he will propose a rider to the Foreign Aid bill before the House Foreign Affairs Committee this week that would create a special State Department desk to help Americans such as Tyler. This section would be able to pressure other countries to release detained American citizens and recommend impounding the commerce of the offending nation, Dornan said.

Marjorie Tyler, who lives in Seabrook in an apartment across the street from her son's high school, said that he was graduated from The Citadel in 1971, served as an Army medical officer in South Korea and Iran, resigning as a captain in 1978 and, after flight training, joined Globe Aero in 1979.