I had a similar issue and found the issue was due to a '''loose battery connectors'''. Yes just that. First I tried all the trick (holding buttons, etc.) But nothing worked.
On last effort I took a scalpel (Yes, I am working in a medical office :-) ) and carefuly pinched the connector of the battery. Inserted it in the phone and immediately the process of resetting the phone that was initiated while I was trying hard with the phone on the begining started and the reset completed successfuly.
The phone is working just nice now.