Why did my phone start doing a download, and it will not turn back on

My phone which is a Galaxy S, decided to do start doing some kind of download...not sure why, or what it was about, but it said not to shut it off. The battery was low at the time, so I plugged it in, and it is was on for at least 4 hours, while the phone was plugged in...eventually the phone just shut off, and now it will not turn back on.

I have tried to remove the battery, and put it back in, I have also tried to hold to hold down both buttons at the same time, but nothing.

Not very happy right now.