My car will not move when its in reverse. What is the cause and how do I fix it?

The last owner said they brought it to the mechanic and they said there wasn't anything wrong with it. The gears don't line up to P, R, N,1, & 2. But it goes into gear and goes forward, just doesn't back up. It worked fine for the last couple of days until today. Any other suggestions? It'a almost like it's locked into place,but parking brake is not on.