It could be the battery totally drained off - just wait at least half an hour before trying to execute anything letting BME do his job.
My suggestion is to leave the N900 under charge at least half a day, then disconnect the charger, wait half an hour, riconnect the charger and see if the yellow led starts blinking. Don't turn on your device!
Then follow http://wiki.maemo.org/Updating_the_firmware#N900

Note that the u key shall be pressed while inserting the USB cable and with device turned off, so:
- start flasher utility on your PC
- open the N900 keyboard
- press and hold u key
- while keeping u pressed, connect USB cable

HTH