Could be 1 of two things: 1. Mirror linkage failed. 2. Something is holding the mirror; sticky foam or the lubrication has dried out. Remove the lens, use slow shutter speed to release the shutter. While mirror is stuck in the up position, very gently pull down on the front edge of the mirror. If it frees up immediately it was stuck on the foam. Might be able to see a mark on edge of mirror from the foam. Hope this helps.
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