SPH200D restart

[ajax010]

I have a SPH200D kype phone that worked fine until I unplugged it and put it away for a while. I now want to use the phone again.
I plugged in and charged the battery, and then installed the base station and phone.
The phone now sits in its cradle saying "unregistered" and inducating a charged battery, but if I either remove it from the cradle or press the menu button (actually says "Reg" not "menu") the phone goes dead and restarts when I replace it in the cradle.

How do I get this phone to work?