another suggestion from the 'so simple no one would forget THAT' side of things. ( I knoe because it's the kind of thing I do all the time).......
Do you have the image that you want to flash in the right directory? and typed/spelled it correctly? Is the emmc and root image freshly downloaded?Has it become corrupt? Maybe try a fresh download off the nokia site, or even a custom one from one of the guys here?

Just some suggestions that can get easily overlooked when flustered because your n900 wont work. I know from experience