back up your phone, and then DFU restore it. if you dont know how to do this, google it. And make sure the screen is completely black. If it has a connect to itunes symbol, it is in recovery mode, which is different.
If this doesn't work, open your phone and try to correct things.
if that doesn't work, thank it for its service, and throw it away. Probably electrostatic damage which is what killed it. Your 3g is an old piece of crud. Treat yourself to a new phone.