iPhone 3G won't charge or turn on

Your battery has failed.

If it only works when plugged into the wall the battery is not fine, it's dead.

ASDazey wrote:

Looking through the forums I have seen problems close to, but not quite like mine. My old iPhone 3G will turn on, saying it has 51% battery life, when plugged into the wall. When removed, it immediately shuts down. When plugged into my computer, it won't do anything. Won't charge, boot, recognize, anything. I can't keep it plugged into the wall, because I have reset it and it needs to connect to iTunes before I can do anything. I am using a Dell Vostro 1500, running Windows 7, iTunes 12.3.2.35 64-bit. Any help would be appreciated. Thank you.

U Can look at the info from Apple:If your iPhone, iPad, or iPod touch doesn't respond or won't turn on - Apple Support

You're welcome. Sorry about the bad news. You can get an aftermarket battery; check out ifixit.com. Normally I wouldn't suggest self-repair, but a phone that old is worth less than the cost of an "official" repair by Apple.