In poor signal areas where the phone gains and loses its network connection often (like inside buildings), the Treo 300's radio gets confused and locks up. When locked up, it draws heavily on the battery and can drain it in just a few hours. Turning on and off the radio by holding the power button does not solve the problem--the radio is locked up and stays on. (you can tell if the radio is in fact locked up, not just out of range, if you turn it off and the red light keeps on blinking away).
To get the phone working again, unscrew the stylus cap and use the pin to do a soft reset on the back of the phone. Your data won't be lost, but it's a good idea to hotsync anyway before doing this, if possible. The phone will reboot with the radio off and no longer locked up. Hold the power button for a few seconds to turn the radio on again, as usual.
Unfortunately, there's no way to keep the problem from happening, and no update from PalmOne that addresses the issue. You can turn the radio off manually when entering an area where you've observed the lockup in the past, but otherwise one just needs to be attentive and recognize when the phone is dropping in and out of coverage and expect it to lock up.
