Have not been syncing since MobileMe was discontinued over a month ago.
Ouch.
Because it involves a MobileMe account, try calling Apple Support and see if they will help you. Probably not, but asking won't cost you anything. Just explain about the lapsed MobileMe account (I'm assuming it was a free promo account) and see if they can help you get your lost contacts back.
You can try a power cycle and soft reset first if you want. I'll post instructions below, but I think the odds of this working are slim.
Now, if Apple can't help you:
If they've really disappeared from you iPad, and you've got no sync service and they are not on your computer, then they are probably gone for good. If you have them stored elsewhere on your computer, we might be able to figure out a way to re-import them to the iPad.
I'm guessing you dropped MobileMe because you didn't want to pay for it. So, if you don't already have an online account to sync your contacts I recommend you get a free GMail account from Google. That will let you have a backup of your contacts the next time (hopefully never) that something happens to your iPad.
Here are the instructions on how to set it up on your iPad with contacts. The normal setup will only sync mail and calendars.
Set Up Your Apple Device for Google Sync : Mail, Calendar, & Contacts - Google Mobile Help
And while you are at it, delete the MobileMe account on your iPad. [after you've talked to Apple] I'm guessing that sometime after it expired it tried to sync, and since the account wasn't there (or got emptied to free space) you contacts went away.
Good luck.
When things start acting a bit weird, it's a good idea to completely shut the iPad off and restart. You should probably do this at least once a week (my unsupported opinion on frequency.)
Just hold down the power button for about three seconds. You'll get a power off slider. Go ahead and slide it off. Wait until the iPad completely shuts down and then restart it by holding down the power button for about three seconds again.
For more serious problems, when shutting it off doesn't work, you can do a soft reset. Hold down the power and home key for about 10 seconds, or until the Apple logo appears. Let go and wait for it to reboot. This will take a few minutes, so be patient.