Some things to check/try.
Plug the iPad into iTunes, select it, and go to the Info tab at the top. Make sure that Contact syncing is not selected. It should say below that you contacts are being synced via MobileMe.
Log onto MobileMe via a computer, and make sure there are some contacts to be synced. It is always possible something happened to remove them. Address is me.com
On the iPad. Go into Settings >> Mail, Contacts, Calendars and select your MobileMe account. Make sure the contacts switch is turned on.
If it is on, turn it off. Go back to Contacts and make sure they all disappeared. Now go back to Settings and turn it back on. Check the Contacts to make sure everything is working again.
More drastic, but perfectly safe (as long as your earlier check to make sure all you stuff was on the me.com site was good), delete the MobileMe account. The re-add it. I've had to do this occasionally when I played around with the settings too much. Things got stuck until I reset it this way.