Did you press the "u"-key at reboot?
Try to remove the battery, start flasher on the pc, hold the "u"-key, connect the usb-cable, and insert battery, while holding "u" some seconds.

The N900 then shows no backlight but a nokia logo, and a usb symbol in the top right and it stays at that state. In Windows 7 it is rocignised as "Nokia N900 (Update Mode)" in the Device Manager.