You'll probably have to reflash the phone.

As has been suggested, search for the Wiki as regards to doing this, download Maemo Flasher, the correct firmware for your reflash and away you go, it's all explained in the Wiki.

Tip, plug usb cable into pc/laptop, slide open n900, hold 'u' on the qwerty keyboard and then plug in usb cable to phone, after a few moments the phone will be detected, run Maemo Flasher and away you go.

It's all explained in the Wiki.

Hope this helps.