I have just suffered the same problem. I was on the phone to my Wife, tried to increase the volumne and nothing changed. Then there was no display even though I was still in call. After call phone was dead, nothing.
A google search brought me here and I tried the hold all buttons. It was hard to hold down the whole volume button at first but once I did the phone vibrated once quickly followed by three more vibrates and after a few more seconds came back to life at which point I let go of the buttons.
All my contacts and settings were held on the phone.