My iphone 4s suddenly lost all the contacts. What to do to recover them?

if you were using icloud for contacts open settings>icloud and turn contacts off then back on then restart your phone.

If the contacts are missing at icloud.com when using a computer's browser, then they are really gone. Your only option is to restore the device from an icloud or itunes backup.

For icloud...

http://support.apple.com/kb/HT1766?viewlocale=en_US&locale=en_US

Go to Settings>General>Reset and tap Erase All Content and Settings. This will erase your device. Then you will go through the setup screens again as you did when your device was new, and when given the option, select Restore from iCloud Backup.

This link gives another overview of backup and restore:

http://support.apple.com/kb/HT4859?viewlocale=en_US&locale=en_US

Kick yourself for failing to use the phone as intended.

You don't have any backup of your phone?

The OP already stated there was no backup

What is the OP?

Well maybe they don't know they have a backup - for example if the OP upgraded to the latest iOS using iTunes there would be a copy there.

Originating poster

The same thing happened to me twice. Suddenly all my contacts were gone.

The solution for me was to go to Settings>iCloud>Turn Contacts On.
Then I went to Contacts and I noticed there was a loading icon at the top (beside the wifi/3g icon), so I let it load. Slowly my contacts started coming back. I was looking at my favorites and I could see the names reappearing. I just left it for about half an hour (using other apps in the meantime) and it was all back to normal.

I hope this solution works for you and others as well. Like this if it does. 😀

try this Recovering iCloud contacts, calendars, and bookmarks from an iTunes backup of an iOS device