iPhone will not turn on! Shows Apple logo then fades out?

[nugget77]

I have an iphone 3g used, jailbroken. Worked fine till last night, left it in car, woke up this morning and was off. I plug it into car charger and now apple logo went on, then fades out and nothing adter 30 seconds. I went to store and guy said to reboot in itunes. I plug it into my computer and NOTHING! wont detect it or even give the apple screen. I plug it into my car charger, hold home and power button and I get the plug into itunes screen. As soon as i unplug it to connect to laptop itunes, the phone shuts off. Help! My business is on this phone. When it is plugged into i tunes, i just get a faint black screen. I only get apple logo when plugged into car charger.

 

[testcard]

Have you tried charging from a mains charger?