There are two wires connected to the back of the mirror. I had this problem once. At first attempt I did break the mirror on one side but I got it fixed on the otherside when I made some sense!!
Here what I did:
I angled the mirror into the upper inside corner while maintaining pressure, I used flat screw driver the first time (DAH)! and for the second time I used my wife's wooden spoon to pry the opposite corner outwards. I did Wrap it with a towel for protection this time. Once it pops out you should check to see if these wires are connected. It pops right back in.
Warning: I did break it the first time I dare you not to!!!!