Laptop not booting, 00 error on screen

My XO-4 has power but won't boot up. If I hold down the game buttons while turning it on, it shows this 00 on the screen.

Otherwise i get just a blank white screen. It booted up normally a few times before but now only the power button seems to work. I don't know this laptop's history as I got it second hand.

UPDATE: a power refresh had no effect, changing the clock battery didn't fix it either. I got the laptop to boot up once, to the hardware self-test (which it passed). But then the 00 screen came right back.

After some research I suspect a firmware problem. One of my comments below was overly optimistic - OLPC's own wiki says the 00 screen is an "early" error code:

== Early Error Codes ==

During the early boot process, CForth initializes a tiny frame buffer in SRAM and uses it to display error codes on the display. In a normal boot, all you see is a flash of white. But if something goes wrong early on, a large blocky two-digit number will be displayed.

Most of those numbers come from OFW - they correspond to the "Msg #: 41" numbers seen when compiling OFW. CForth kicks off the process but OFW follows up. CForth is responsible for making the numbers visible if you press check during the boot process or if OFW doesn't take over the display within 8 seconds.

But what error does "00" mean?