Given the locking mechanism on most cash boxes, it is highly unlikely that the keys are locked inside. (It's possible with some models -- but the lids on most cannot be closed if the lock is in the "locked" position.)
Unfortunately, beyond that, you'll find that people here aren't going to be able to help you much. On a practical side, we don't really have enough information to know what type of lock you're talking about. Aside from that, though, instructions for opening safes, cash boxes, cash registers, etc., simply aren't discussed on the "public" sections of this site... there's simply too much risk of someone using the information inappropriately.
I hope you understand the rationale... and, if you're interested in picking up an interesting new hobby, that you hang around, read a bunch, and start to explore hobby lockpicking.
