http://wiki.maemo.org/Documentation/...mo_Flasher-3.5

1. Charge the device if you aren't sure whether you have battery more than 50%;
2. Download and install flasher for your OS from
tablets-dev.nokia.com/maemo-dev-env-downloads.php
3. Download the latest Maemo 5 from
tablets-dev.nokia.com/nokia_N900.php
4. Follow the instructions from
http://wiki.maemo.org/Updating_the_tablet_firmware

Most, if not all, programs you installed previously will be gone. But your photos and videos and documents will still stay here.

P.S. I had another problem: after the Nokia logo the dots ran indefinitely, and I wasn't even asked for pin code. Now it's solved.