It might be same type of problem that I am experiencing, posted a question here today, I will quote my question bellow.
But in short just remove and reinsert battery and see if it does not work again.
A bit interested to see if you have the same problem as I do, I will check back for your answer later.
My question here entered today:
P500 with firmware v1.0.
Works like a charm but twice now it have not reacted on the power button when in off state.
The only way to get it working is to remove the battery and reinsert it and it will then power up nicely.
Battery is fully charged.
Is this a known problem that will be fixed in future firmware update? or send it in for repair?
No problems otherwise with this nice product.