My contacts disappeared after I added manually another contact.
I reboot my IPad but nothing comeback, everything in my contacts is gone.
Any ideas?

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.

Same here. My iPad erased all my 2000+ contacts today. It's an iPad1 I bought early April this year, 16GB 3G. OS is the current one ("4.3.1" also ca 3 1/2 weeks old). Synched it today. Disconnected it from the computer, then changed one contact (added fax number), confirmed (pressed "done"), now all contacts are gone, even after 2 re-boots (held both buttons to turn off and re-started).

Not using MobileMe or other online services, it is no import or export issue from other programs either, as I just use the Mac Contacts address book and Mail, just as on my computer.

Gravis Mac store service center in Berlin had no clue what to do, how or why it happened. They would also not exchange it for something else. How can I trust this thing now to keep it's info on the road, when I actually need it? I guess the only safe way to store contacts is to use a paper based address book...or??

As twerppoet has already mentioned use gmail, yahoo, hotmail, or any other exchange based system or mobileme (if you want to pay). Then they are backed up by your provider automatically over the air.

Its also very handy because all your contacts are availble to you on any internet connected device, so you don't have your ipad but need to send email or even just find a number, just find your nearest internet cafe and away you go.

Well, a 500EUR device featuring an unreliable address book "yes, I can give you that number, hang on, oh, is there an internet cafe or WIFI anywhere? I need to recover my contacts quickly..." nope, not handy at all. Quite scary indeed. I can't trust this thing anymore.

With a freaking 5$ paper address book, I am still on the safer side - and that won't record all my movements btw... THX for the suggestion tho.

There may be an issue with contacts, if they are not assigned to a certain service, but I don't think the iPad will delete contacts, when they are actively synced to a certain service.

I have been importing contacts from Gmail, Hotmail, web.de & Nemo onto two iPad's and than syncing all contacts back onto Google and the contacts are always there.

It may at least be worth a try.