It is possible that the phone has hung up because there are errors in the software. So to reset remove the battery and the memory card from the phone. Using your PC , scan the memory card for any corrupt files or media files with virus- it is possible when they are downloaded from unsafe sites. If so remove or format the card. Copy only valid files to the card and do a scan before it is replaced. Now replace the card and battery and switch on the phone. If the phone hangs up or is slow when it is switched ON , then it is possible that the fault is relative to the software and will need a reinstallation. Possible hardware issue cannot be ruled out.