Can you remove the battery? If you can, try and remove the battery and then put it back in, do not press power button. If the phone turns on without you pressing the power button, then it is likely that the power button is bad. If this doesn't resolve the issue, I would try a new battery. If that doesn't solve the issue then see if you can get the phone to boot into the recovery menu and wipe the phone.
Start by switching off the cell phone by using the Power key.
This should be the instructions for the recovery menu:
Then press and hold Volume Up + Power button until you see an Android logo on your screen.
Select using Volume buttons option "wipe data/factory reset", and confirm that with Power button.
Select "Yes--delete all user data".
Let me know if any of this works and if not I will try and help further.