have you tried plugging it into the charger? Does it start charging (flash green light)? If yes, wait until a minute or so until the "charging" text appears and then, when it's charging, try to start it again (still plugged in). There's plenty of threads here about the N810 not booting up - taking out the battery for a while and then plugging it into the charger and then booting works for me. Good luck.