It's something you find no support for. Even the issue you have is rough to find support but for years my troubleshooting is to try my other PC and working cable. If the phone fails on the other PC and the other iphones work I usually find it doesn't work at the Apple store either. What happens next depends on the warranty status.
I have my daughters old iphone 3g and she Reset it from the phone,but i do not know if it needs resetting in itunes as nothing sticks no music plus no Apps will download as the Firmware for the 4.2.1 on a 05.15.04 modem Firmware it says in settings. Evertime i try to load say a basic App like Facebook it says on the pop-up that i need a different Firmware basically for any Apps now,i hear that Apple are basically done with the 3g and it's being made void so now im looking for the best way to jailbreak this brick to get Cydia and all things back to some normality. Can anyone please help me i have tried the jailbreak me and it does not even come up as chance of Jailbreaking this stubborn little brick,all i ask is a good dev site where i can simply d/load some redsnow or yellow snow for all i care for this measly little thing.....plz...plz...plz...help the numbers are all there and thanks in advance i now need as a good reply to Jailbreak now : (