Don't know if this will work but try and start the phone in safe mode.
I realize that the phone is not technically ON per se, but I'am hoping that if you follow the directions of holding the Power button to shut down the phone that by holding the Power OFF menu option it will then restart in safe mode.
If it does start OK and boots all the way to the Home screen, then a downloaded app is the cause of the problem. The trick is to find out which one.
You'll have to uninstall each downloaded app one by one and test the phone between each download until it finally starts OK 'normally'. The last app that you uninstalled will be the culprit.
Try by uninstalling the ones installed or updated just before the problem started.
If it still doesn't start OK in safe mode, did you try the master reset with hardware keys method of performing a factory reset?
Be aware that a factory reset will erase all your data and downloaded apps. It will restore the phone to its' factory default condition (or last system update condition)