iphone 4S - contact names deleted

Weirdest thing. Just saw that although the phone numbers are there on Favourites and Recents and the contact list is complete, all the names attached to the numbers on the callers in Favs and Recents are gone. Just a load of numbers which means nothing. Anyone able to explain (in words of one syllable for preference) how to reinstate the callers' names? I'd be so grateful.