there are a few things this could be, depending on how long it has
been sitting.  lets assume the problem is minor.... usually is.  put
it in 1st, clutch in to nutral, double clutch it and go to reverse, if
this doesnt work put the clutch in and put it as close to reverse as
you can, let the clutch out slowly..... does this gear start to grind
at all?  second, check linkage for any obstructions or rust.... i had
a friend who had a bottle cap get in the center counseland it forced
itself in the rev. linkage and did the same thing your explaining.....
hope this helps, let me know