new appliances or moving home phone base station can easily cause issues like that. even a fridge/microwave etc getting old and starts leaking rf noise in to the electric lines.
after making sure that no new moterised applicances have been added/moved see if wireless signals improve while you move from corner to corner in the room that used to work. there may be some apps out there that allow you a graphic visulization of signal noise/quality. if you find one use it to see if you can track down the source of the problem.
also did the ap recieve an update from your isp? if its part of the modem they may have updated something that doesnt work well with your pc. try setting the ap to different channels and see if it works better.
Forgot to mention RF noise can travel through the walls using the home electric lines. x10 light controls and such use the same principle to allow any outlet etc to be turned on remotely. if a wire runs through the wall of that room to say an attic that just started getting used and now has a window ac unit, that ac could be the cause of the problem even though it is no where near the room in question. Its all a matter of how the wires are run in the building. Had a user site that was in a strip mall and there were some huge mainline power conduits on the outside of the building along 1 wall. they could not put a wired ethernet connection near that wall due to the power lines outside the building.