Either the selector fork for reverse has moved on its shaft or the shaft it is fixed to has moved beyond it's normal movement and potentially the detent plungers are stopping you selecting reverse. Or the shaft isn't moving enough to engage the gear.
When you say you can't engage reverse what is happening? Does the gear lever not move into the correct place or if it does is there any gear grinding noise? Were you able to select reverse with the engine running at any stage?
Anyway, as Mike suggests, too early I think to conclude you need a new box. Maybe the tunnel is obstructing, maybe something easy to fix inside. Investigate!
They're pretty easy to open up and put back together again - take a look.