first off, plug the n900 into a pc via usb, this will mount it as mass storage, transfer any backups or anything valuable to the pc. Just in case.

then use the power search to search for the updating wiki.
download the firmware you require (go with PR1.3 if unsure) and flash your firmware.

if this works, brilliant, your good to go. you should still have all messages, phone numbers etc in tact.

if this doesnt work, you may have to flash the eMMC aswell, that will erase all messages, numbers etc, thats where the backup you (hopefully) transfered to your pc comes in.

transfer that backup back to your nice fresh working phone and use the backup utility to restore it.