I had this happen for the first time not too long ago. I've had my 505 since Christmas 2008.
I started pushing buttons and trying to turn it off and on. Nothing worked. I let it set to see if it would do anything. Then I plugged it in to charge it with the wall charger for a couple of hours. Still nothing.
I finally gave up and tried the reset for the first time.
Nothing. I tried the reset and then slid the switch. Nothing. Then about 20 minutes later it was working. I still don't know what happened, but it works.