I´m not expert but there is little that you can do if internal memory is faulty.

Do you use external sd inside N900?
Have you overclocked N900?
Battery off, give it 10 minutes, put it back and see what happens.

Btw.
http://talk.maemo.org/showthread.php?t=63975
Has fsck but now that your device is booting it´s probably not helpful. Try to use power search and search for fsck and read those threads completely before doing anything.