Most likely you have a custom theme installed and that is why your camera doesn't work. For some reason no one makes a theme with a working camera. Just restore your phone or get rid of the theme and it will work.
This used to happen to me alot... most likely u have to many apps running at the same time or running in the back round... how I solve it is by just resarting my droid2 by turning it off and on again... hope this helps!...