That is an odd issue for it to not work on neither 240v or USB charger. Sounds like you might have a dodgey mini USB - You tried removing the USB lead in PC, putting it in another port then attaching the N900 again? Try that, it's a long shot. Also try wiggling the cable whilst it's in the port. the port on my N97 was a bit flaky and doing this seemed to bring it to life.