I had charged the 8120 the day before. Put the phone in my drawer and did not use it for about 15-16 hours and when I came back it was completely dead. None of the buttons were responding.
So finally I took the battery out and put it back in and the phone started to power up. I also saw a few red led flashes for a few seconds when I took the battery out and put it back. It took 2 mins to power up with security updates etc.

I am puzzled why it happened as the phone has over 3/4 of charge in the battery and I have not done any SW updates for about 5-6 months.

I have v4.5.0.110 on my Pearl.