Help my mytouch 3g doesnt work need help

Hi, so my phone was about to die didnt make it home in time to charge it. I dont have the original charger for it so the only way I can charge it is with USB connect to my PC now here the part that I have never seen before. The phone turn on for like 3 sec ( showing the mytouch 3g logo green background) than turn back off, it just keep repeating this over and over, I would take out the battery out and have it connect to my PC to see if it turn on without it and it doesnt work, I would try turning it on without the battery but being connect to the USB and it doesnt go on, I would take the sim card out and micro SD and notting, It doesnt turn on without being connect to USB with the battery on, I left it connect to my PC for 4 hours so it can fully charge (it still does that on and off thing ) and notting I cant even tell if its fully charge or not.
Please can anyone help me or has this happen to anyone before?