You have likely installed some application that is interfering with phone. Start with latest application you installed, then gradually uninstall all application you added to phone till problem is fixed. To view and uninstall applications, go to Start/Settings/Remove Programs.

If you have no luck with this, then Hard Reset your phone. You can do a Hard Reset which will reset your phone to its original factory settings. Any applications or personal data you put on the phone since owning will be lost. You can recover personal file if you have backed them up, of if you have License 'Key' numbers you got when you purchase them. Some App suppliers will have you an record and will email your Lic # if your email has not changed since purchase.

To Hard Reset, turn off your phone and then turn it back own while holding down both left and right soft keys till you get a screen asking if you want to Reset it. Another Hard Reset method, if you can get to Start Menu/Accessories folder when phone is powered, is choose Clear Storage and clear the storage. This may not work on some phone models, but note this method will also reset your phone to 'as purchased' state, with all stuff you added since removed.

Note a Soft Reset is the same thing as just turning phone off then back on as normal. It does nothing but re-power your phone, but a Hard Reset recovers your phone operating system from phones MFG ROM and resets it back to original as purchased state.