The japanese manual says: "Do not turn off the power or restart while data is being written to the flash memory. Data may be corrupted or the Nexterm may be corrupted". The numlock key responds to being pressed, and there are a couple of green leds turned on inside the unit, I'll open it again soon. Here's a picture of the board the last time I opened the Nexterm:
Is there a backup battery somewhere on it? I'm thinking if you cut power to the memory it may reset things. My Jornada totally froze when I tried installing a random software that wasn't for it. I removed all the batteries and let it sit for a while and it reset everything back to square one. Lost my files, but at least it was working again
I haven't tried it again since last time. If it doesn't start anymore, I'll see if it's possible to find jtag pins on the board and plug an interface to dump the flash that way...