I too have had this problem with an N800.

The first time it happened I didn't pay much attention to what I'd done beforehand, but I did know that the charge was very low, so I recharged it and it was fine afterwards.

The next time it happened was after downloading some maps to the external SD card (actually an MMC card, but that shouldn't matter). Again, the charge was low, but certainly sufficient to power up the device. I tried all kinds of things, such as removing the battery and both SD/MMC cards and nothing worked. Strangely what seemed to cure it wasn't a battery charge at all, but simply sliding back the clip on the internal SD card. After that, the unit would start up fine without a recharge as long as neither card was plugged in. After running dosfsck on each card, the unit would accept the cards again despite dosfsck claiming to have done nothing.

Since then, I haven't had any further problems switching the unit on, but I have steered clear of writing to either SD card and I don't have swap enabled.

Sorry if this clouds the issue, but I thought I should share my experience.