the link shmsnh has given you is a "if all else fails" method, and a great link to have, please note though, this does require that you are on a computer that is running windows7 or higher, if you arent running one of those versions of windows your SOL with that link, I didn't see a post by "Rich" here, what method did he recommend? in case he doesn't respond i might be able to further you along, also there is another trick that might work, some have called it BS and in truth i thought that to when i first stumbled upon it, however in desperation i gave it a shot and was actually surprised it worked, i have since tried it on 4 other phones 2 920's 1 925 and 1 520 it worked on the 920's and 520 but couldn't get it to work on the 925, so take it for what its worth, the method is as follows:
with phone powered off, press and hold vol down and connect wall charger, when exclamation mark appears press in order, vol up vol down power, vol down, phone should then reboot.
like i said, alot of people have said this is BS, who knows, maybe by coincidence i just got lucky 3 out of 4 times, but it has worked for me