Why did my Computer freeze?

I built my own computer a few weeks ago, and everything was fine until about a week ago when while surfing the web it suddenly restarted. I ran PC Wizard, and everything came up normal, I've been monitoring all of the temps and nothing seems abnormal. The highest the case temp got to was 42 C. The GPU runs a little hot when playing games, the highest I've seen is low to mid 50's (c). It has happened only while surfing the web. Could it be chrome? I doubt it. But today I was surfing the web, and the screen locked up. I waited about a min. to see if it would work itself out...it didn't. I heard a loud long beep coming from the mobo and promptly shut it down. I then turned the computer on and everything was fine. I can't imagine this being a software problem because I only had Chrome open, nothing else was working. I don't think it's the PSU. I have all the parts to my computer listed on NEW EGG on a public list titled "My first hand built PC". Any help would be greatly appreciated.