HTC XE turned itself off and won't turn back on

I charged my phone overnight for at least 6 hours. It was working fine when I checked my emails first thing this morning. Then, the phone restarted itself (not uncommon!). Since then, I can't turn it on. I have taken the battery out and left it out for around 2 hours; it still won't turn on. I have tried to turn it on in recovery (holding the down volume button and the power on button) and still nothing happens. Any help would be greatly appreciated. Nick