My contacts have disappeared this morning, I backed up yesterday but dont know where the back up is

Where did you backup to icloud or itunes? see link to restore from backup http://support.apple.com/kb/HT1766

the iphone is designed to be synced to your computer. You should have been syncing your contacts with Outlook, Address Book for example from day one.

Have you failed to do this?

"* Your contacts are part of the backup to preserve recent calls and favorites lists. Back up your contacts to a supported personal information manager (PIM), iCloud, or another cloud-based service to avoid any potential contact data loss."

Learn more about what is stored in a backup.