Help Galaxy S will not stay on


I have looked through the forums, and haven't been able to find anything that is exactly what's going on with my phone. I have a Samsung Galaxy S GT-I9000 (union wireless). This is a "temporary phone" until they fix my other Galaxy, and it is a Vodafone it appears.

It worked fine until this morning. My phone was dead this morning and when I plugged it into the charger it turned on until I could see my lock screen, said it was loading everything, and then turned itself off and then back on again. It has been doing this for over an hour- off, on, off, on. It doesn't appear to actually be charging either because I can't turn it on if it's not plugged into the charger. I have tried using a different charger, taking out the battery for a while, taking out the battery and SIM card, doing a hard reboot. Nothing is working. Does anyone have any ideas? I don't know much about phones when it comes to these things so any help greatly appreciated!!