I have a Samsung Nexus S which I bought off my friend. A system update popped up and I hit accept without even thinking about it but the phone said I had to charge it so I did and left it for an hour or two. When I came back and went to turn it on it just went midway through the reboot and turned off again, then without even touching it it would repeat this process over and over. I have absolutely no clue what to do - how can I get it booting again?
I'd suggest you start here:
The part that is relevant to you, assuming your device wasn't running a rooted custom ROM before, is 4B:
Tried to include as much as possible for posterity, but the files you need and so on are in the forum thread anyway so this doesn't really apply if the forum thread would go down.
If you have any questions, please comment and I'll update my answer.