From an old comment column on google:
No guarantees
I recently inherited a Canon AE-1 camera and after shooting and unloading one roll of film, the film advance lever and shutter release button no longer function. I've replaced the battery and it still is not working. The shutter release button presses down, but nothing actually happens and the film advance lever moves but cannot advance anything. In addition, when you look through the view finder, it is dark.
If the mirror is locked up, it most likely is a mechanical problem, not electrical. The first thing to check is the black plastic rim that goes around the shutter speed dial on the top cover ( it goes around 90 degrees of the shutter dial). Gently try to move it. It should not pull up at all as it is held to the top cover with several heat staked plugs. If it moves up a small amount, one of the plugs has broken off and fallen into the wind mechanism. I have seen many AE-1's with this problem when the mirror is locked up.
I had similar sounding symptoms on my brought-back-from-the-dead Canon A-1. Out of several shutter releases, the winder wouldn't move and the viewfinder turned dark in a few of them.
It turns out that the reflex mirror was getting stuck at the up position. I could force it down (using the clip of a ball point pen cap to pry the mirror away from the up position), and the mirror appeared to then retract to its normal position and the camera would function again, until the mirror locked up again.
The solution in my case was a touch of a lubricant oil to the hinge of the mirror driving arm on the side of the aperture box. In A-1, while looking from the front, the hinge is on the left side wall. The camera has been working flawlessly ever since.
My AE1 is doing the same - mirror is locked and the advance lever moves and is jammed. The reference to the black plastic rim - mine is separated and I do not have the tools to pull the lever off.