In my experience, the cause is normally an app that has been installed. The normal culprits are home screen replacement apps but others can also cause the problem, like phonebook apps. Have you installed any new apps? if so, uninstall them and see if it fixes your problem. If it doesn't, try a factory reset (settings---->sd card & phone storage----->factory data reset). Make sure you sync your contacts first. Any pics, videos, etc will not be affected as they are stored on the sd card.