i had this problem, amongst others stopping my phone. i pulled the sim card out, then rebooted the phone. the phone then showed it was updating all apps i let it boot up and it seemed to work. pulled battery and put sim card back in. and all ok so far. i have sent my phone 3 times away to vodafone for this repair, with android.clock stopping, android.phone stopped working and the phone actually crashing on the samsung screen and not even booting up. each time all they do is a factory reset. like duhhhh, i have never thought of that (sarcasm). i am hoping that doing this, will give my phone some sort of permanency in it's repair. i have not rooted the phone, it is stock.
Vodofone refused to replace the handset even though it had been in so many times for repair. word of advice. steer clear of vodafone. i am cancelling all my contracts with them.
sorry that went off topic, but i want a phone that works and vodafone will not help you in that matter.