To fix this problem, you can try several things. If a reboot doesn't solve it, you can perform a factory reset. To do this, go to Menu > Settings > Res. factory sett. There are two options available, to keep your data or to remove it. It's advised to wipe all the data. If this doesn't help, try to update your phone's software. You can do this using the Update utility that you can find on the following website, or you can do this directly from your phone. Check the mentioned page for update instructions.