look i have created a post about this before but i never finished it because every one said there was already a tutorial.

There is a very exact process you must go thro to reflash your n900.

The chances are that you cant get to r&d mode because your battery is to low. I didnt belive this untill i had the same problems as you.

As i already stated you can not charge your battery through your phone in this state you must use an external charger or x5800.

check this thread http://talk.maemo.org/showthread.php?t=43067 i have a post half way down the page it should help clear up the process you need to go through. It is unfinished and probaly will not get finished inless i get more thanks.

for the unfinished sections see the reflashing wiki to fill in the blanks.