The first thing I would do is try pulling the battery, leave it out for about 2 minutes, then put it back in. See if this will reset the device.
NOTE: Suggestions listed below here can result in loss of data, so please use wisely.
If this doesn't work, then I would try locating the "Reset Button" on the phone. It is a small pinhole. Use a pen or other object to carefully depress it which should do "Factory Reset" of the phone. Please do not use an actual pin or other sharp object as you risk damaging the device.
Last option is a "Hard Reset".
NOTE: Doing this will clear the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up...