This has happened to me twice now. The thing goes dead, has a blank screen, and becomes totally unresponsive. Each time I have assumed that the battery has given out or it's not connecting properly with the recharger.
BUT! It just needed to be rebooted.
Press the mode and the lap/reset button simultaneously for a few seconds and voila, it comes back to life!
It didn't even lose any data.
I don't know why it occurs, or what prompts it to go dead. This fix is listed in the manual, but it's not obvious to find.
It's become increasingly obvious that I've become addicted to my Garmin, too, since I really don't like to run without it any more and was even thinking of buying a new one when I thought this one was dead. I want to know my current pace, average pace, and distance run. All the time.