Well, it doesn't need the sim to power up, but it does need the sim to do anything once it's powered up. I never understood why that is. It would be nice to access the PDA features without a SIM. :?
Sounds to me like you're battery is acting up. I would pull the battery out of the phone for a good couple hours. Then put the battery back in the phone and plug it in - Don't turn it on. Leave it to charge for a good couple hours - or 4. Don't try and turn it on, or anything. Even if the charge light doesn't come on, just leave it plugged in. Return after a considerable amount of time, and try turning it on while it's still plugged in.
Give that a try and let us know how it goes. (It's worked a number of times for me.

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