This seems to be a common fault on the recent generation of Nokia phones.
A master reset of the phone using the process below usually solves the problem;
1. Performing a master reset will wipe all of the your personal information from the device so you should do a back up or copy out any information you wish to save before continuing with this process. You may be able to do this on the memory card or your PC.
2. Remove the memory card. From the phone pad, enter
*#7370# 3. The phone will ask for the ‘Lock Code’ which will be
12345 (unless you have changed it).
4. Your phone will be taken back to factory restored condition and then you will need to enter the country, date, time etc. as you did when they first switched on the device.
Hope this helps