I can only assume he is talking about the "jockey wheels".

your rear deraileur has 2 cogs that the chain snakes through. although it would be kind of a weird failure (the bolt sheared off and the cog fell out??) it is an easy fix if thats what the problem is.

To add some comfort I will say that even if the rear (or front) der is damaged beyond repair you can get a replacement fairly inexpensively. As long as the rest of the bike is what you want, a broken deraileur is not a show stopper.