Hi,
Try a hard reset using the phone's factory restore feature.
''Back up your phone first'' using the phone's backup feature because a factory restore will erase all your downloaded apps and user data. It will return the phone to a factory default condition.
Ensure that the battery is charged before you perform the reset.
Once reset see if it now works. If not it most likely is a hardware problem.
If it now works, restore the phone with the backup you made earlier, using the phone's restore feature.
If it still works then the reset has resolved the problem.
If it doesn't work after the restore then a downloaded app may be causing the problem. The trick is to find out which one. You will have to uninstall your downloaded apps one by one and test the phone after each uninstall to find the culprit.