Iphone wont turn on !

Hello,
well my iPhone (1st gen) wont turn on, just after i downloaded something from cydia. Ive tried pressing the home and hold/off button and when i try to sync it on itunes it wont sync (like it wont show the name and stuff on the side bar). And after i pressed the home and hold/off button the apple sign would show for like seconds then it turns off again.