Do you sync with a service like Google, Yahoo, or MobileMe. If so go into Settings >> Mail, Contacts, Calendars and open the account with your calendar. Make sure the Contacts info is selected. If it is, try turning it off, then on again. That should force a resync.
Other things.
Make sure there is nothing entered in the search box. The Contacts app will only list matching addresses, and you have to clear the search field manually.
Second, tap on the Groups ribbon/button at the top left. Depending on how you set things up you can have more than one address database. If you've accidentally created an empty one, it might be selected. Or an empty group could be selected.
Failing that, have you synced your iPad lately (but before you lost the contacts)? If so you've got a backup you can try restoring from. You will lose any recent document changes, like pictures, contacts, events in the calendar, or documents you've worked on in other apps. You'll want to save them off the iPad first. But that should be better than losing your whole contact database.
Do this only if you have a recent backup.
Open iTunes without connecting the iPad. Open iTunes Preferences, go to the Devices tab, and turn off automatic syncing. You can also look at your last backup file here and see when it was done.
Connect the iPad to iTunes. Right click on your iPad where it is listed under Devices. Choose Restore from Backup.
