I've seen a few discussions about this recently and the suggested reasons and solutions show a lack of understanding of general principles. So here's a few reasons of why a mirror may stay up.
1. Mirror is stuck to the mirror foam.
This is a myth, often repeated but, I imagine, never actually seen. I've never seen it in the hundreds of cameras I've repaired. Even ones with very sticky foam, I've never seen a mirror actually stuck to it. Let's move on.
2. Mirror hits the back of the lens.
If the mirror slips down in the mirror carrier such that the bottom of the mirror overhangs the carrier, it 'might' hit the back of the lens. It depends on how much the mirror has slipped and how far the lens protrudes into the camera. It may happen with some lenses and not with others. It may happen with some lenses only if they are focussed on, or near, infinity as the lens moves further away form the mirror as it is focused closer.
There are two possible scenarios if the mirror hits the back of the lens depending on how high the mirror gets before getting stuck.
a) If the mirror rises sufficiantly for the shutter to fire, the shutter will open and close normally but the mirror will not fall after the shutter closes. The camera can be cocked again but the mirror will remain up.
b) If the mirror rises insufficiently for the shutter to fire, the shutter will remain cocked and no exposure made. The camera will become locked.
In both cases, the camera works normally if no lens is fitted.
3. Shutter fails to close.
The closing of the shutter triggers the release of the mirror. If the shutter fails to fully close, the return of the mirror doesn't get triggered and it stays up. The camera, usually, can be cocked again and the mirror will typically return back to the down position.
Depending on the camera and the type of shutter, sometimes the shutter can be pushed to make it close fully and the mirror will then fall. Try this at your own risk.
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