Used to hear about this every now and then with the earlier versions of iOS 4.x. If you are on iOS 5 this would be the first time I've heard of it on that version.
The usual solution is to make sure you are syncing with an online service like iCloud or GMail. Because they update constantly (as long as the iPad has internet) the chances of loosing anything goes way down.
If you are running Windows Vista or newer, and iOS 5.x on the iPad you can keep both synced with iCloud.
If either the computer or iPad has older software (and you can't or don't want to upgrade) then you should be able to sync both to Google's contacts. Use this link to set up the iPad.
Setting up Google Sync with your iOS device - Google Mobile Help
If you use either of these, be sure to turn off the direct syncing in the Info tab in iTunes, or you'll end up with duplicates and other problems.
As far as getting your contacts back, if you've been doing backups you can try restoring the iPad from your last backup via iTunes. Hold in mind that you'll lose anything newer than that backup, and not just contacts.
Other than that, I know of no way to recover contacts that disappear off the iPad.