Iphone 3gs locked after jailbreak

After the operation of jailbreak that seemed to have been successful my iPhone rebooted and since last night it has the apple icon at the center of the screen. Doesn't respond to any command, it is not seen by Windows 7 (connected by usb) , nor by Itunes. Doesn't enter DFU mode and doesn't react pressing the volume up button. Only the apple and, from a couple of hours, the signal that the battery is running low.
What to do? I'm losing hope ...
Forgetting: usb cable is new and both buttons sleep and home were ok till yesterday
Thanks for the help!