Hi:
This could be because of a few things.
If you are using Crystal XI, and using to monitors, Crystal can "lose" your field explorer. To fix try using one monitor, or have crystal run on the primary.
Have you tried the Field Explorer Icon?
Perhaps you have set the field explorer to "auto-Hide". In which case you need to place the cursor over the field explorer tab.
Could be other things as well. If this does not do it for you. Let us know a few more details like... what version of Crystal are you using?