Suddenly- No reverse

I have an 86 Mustang convertible that I drive (minimally) on a daily basis. This afternoon, I pulled out of the driveway to leave and I heard a hard clunk in Reverse, so i placed in drive, stopped, put in reverse again and heard the clunk, only less loud. Put it in drive, drove to Home Depot and when i got ready to leave, the car would not reverse out of the parking space!!!!
I got it to slowly roll back and drove home...but NO reverse. Whats up?:banghead:
Annette

Reverse planetary gear/clutch setup is gone. Your gonna need a rebuild. Pull your pan and see what is in it.