2 days ago i drove my car completely smoothly, backed out normally from driveways and parking spaces without any problems. went to move it this morning and will not go in reverse. it is a manual transmission, and sometimes the gears stick, but they are not sticking. i can put it into all other gears and it works perfectly, but when i put it in reverse it acts as if in neutral. i have a mechanic i can go to, but before i do, any idea of what could be wrong?