I have the exact same problem, only mine will not even turn off. Is there any way to flash it when it is on?
It will say: Suitable USB device not found, waiting * N900: Whilst holding the ‘u’ key, plug the USB back in. Wait until you get a dark screen with the NOKIA logo in the middle and a USB logo in the top right, and let go of the key. 1. If the USB logo does not appear, and the device turns off - ensure the battery is charged by connecting to the wall charger for an hour. * N810, N800 and 770: Take your device, and hold down the Home/Swap button (looks like a little house on the N800 and 770, and two overlapping rectangles on the N810), and while holding down the Home/Swap button press the power button to turn it on. If it doesn't boot and start flashing, make sure the USB cable is plugged in.