All my contacts disappeared from my iPhone 4S for no reason

Check your contacts on icloud.com from your computer. If they still appear there, on your phone go to Settings>iCloud, turn Contacts off, choose Delete from My iPhone when prompted (they will still be in iCloud), then turn Contacts back on.

Open Contacts, tap Groups and make sure you have checked your iCloud contact groups. If you have, and they still don't appear on your phone but do appear on icloud.com, try deleting the account, then signing back in. Before doing so, save any photo stream photos older than 30 days to your camera roll (open your my photo stream album, tap Select, tap the photos, tap the share icon -- box with upward facing arrow, then tap Save to Camera Roll). Then go to Settings>iCloud, scroll down and tap Delete Account, choose Delete from My iPhone (your data will still be in iCloud), then after a minute or so sign back in.

You're welcome.

Is this answer specific to iOS 6? I'm having this same problem, but I don't see anything about Groups in my Contacts on my phone . . .

Thanks!

Do your contacts appear on iCloud.com when you check from your computer?

I've posted the info on my specific problem here: https://discussions.apple.com/thread/6071787

I just wanted to know if your answer was specific to an earlier version of iOS. 😉

No, it isn't specific to an earlier version of iOS. Deleteing the account and signing back in can often cure intermittent iCloud syncing issues.

Okay--that's what you're talking about. The terminology you chose when you wrote "Open Contacts, tap Groups and make sure you have checked your iCloud contact groups," didn't make me think that was what you were talking about. I'll keep that in mind for next time, if a restart doesn't fit it. Thanks!

Same thing for my phone except that i hadn't backed upto icloud or itunes but google mail. Anyway to restore them? Please help