well, You need to first figure out what made your contacts disappear.
But if that is too time-consuming or is a dead-end, then here are the only things I can think of...
Have you checked the program on your PC that you sync with to see if your contacts are still there? If you do not know where they sync, that can be a problem. Connect your phone to iPhones, and use the Slide to unlock to stop any attempted sync. Click on your phone in the left column of iTunes and then click on Info (located in the top middle of the iPhone screen). It should tell you right there where you sync your contact to. If it's not checked... Don't quite panic yet. If it is checked, open that program and see if your contacts are there. If they are not... Still don't panic quite yet.
Ok, lets day you haven't been syncing your contacts, or they are missing from the program you sync to. If you have sync'ed your phone at all since getting it (and after you added all your contacts) then you should be able to recover everything. The most recent sync before you discovered everything was missing is what we need.
Here's the hard part. You are going to have to restore your phone. When it finishes restoring, it will ask if you want to restore your profile from the last sync. You can choose the drop-down menu to choose a different sync (which is a must if you have synced the now empty phone since discovering it). Again, try to choose a sync that would be one that is most recent that you would have all your data on.
Once you choose that and tell it to restore that profile, all you have to do is wait for it to finish (watch for conflict error messages. if you walk away and come back in an hour, it might still be sitting there waiting for instructions).
Your previous data should be synced back onto your phone. Once you have verified that everything is back, then sync your phone again and make sure that the programs it syncs to are updated with the missing data as well.
You should be good from there.
One more thing. You might be asked to back up your phone when you start the restore. Say no. There is no point. Also, if you get a message that there is newer firmware available, it is more or less up to you if you want to update it. I personally say to choose no and get everything back in working order before worrying about an update. BUT it's possible that you might have to, depending on if Apple is still signing that firmware or not.