you should go to download sections of this page above. look down and right you should see n 900 click there. enter your IMEI (you can find it under the battery). Then accept trhe agreement download the latest vanilia image and the latest fiasco-os image then install flasher
you can reflash your phone following this wiki http://wiki.maemo.org/Updating_the_tablet_firmware . This will erase all your data so you should use the back up app to back up your settings contacts emails and copy the back up of the device to a pc. (ph the phone is not working you cant my bad)