My Canon AE-1 film advance lever won't budge. The camera hasn't been used in awhile, but I know of no damage that might have caused this. Afraid to force it for fear of breaking something.
I fetched my AE-1 and did some tests. One small disclaimer: it seems my camera’s battery is close to fully discharged; it might have affected the tests I did.
Note that the lever won’t bulge if you’ve advanced it previously. If you need to release the shutter, you can open and close the film compartment and the shutter should activate.
But apparently you need to have at least some minimal charge in the battery for the shutter to work. Even if your battery is old (like the one currently in my camera) you might have some luck if you let it “rest” between shots (or open/close the film compartment).
So, what you’re experiencing might be because there’s no battery remaining.
But if you’re sure there’s no film inside you could possibly ruin, you can try opening and closing the compartment to see if the shutter releases. If not, then you need a good battery to see if your camera is still in good shape.