It could be that your motherboard got fried if you touched the battery connection part on the motherboard with a magnetic screw driver. If you can try putting back the old battery.. IF you can. Did it work right after replacement?
when you take apart an iPhone 3G/3Gs... do not forget to turn it off before you take it apart. it usually in a way "tricks" your motherboard into thinking your LCD is on when it is not, and and when it is all together your LCD is black. so don't forget to turn it off before you start the repair..