Windows 7 Unexpected Shutdown?

So I was on Facebook and all of a sudden My laptop screen went black and then shut down. I have a Compaq Presario CQ60. Then It took like 30 minutes to turn off and I turned it on. When I turned it on it said it had an error of "bluescreen." What is this? It was really weird. Then When i back on the internet I was signed out of everything. My facebook, myspace, youtube, google, and yahoo was all signed out. Usually I'm automatically logged into everything right when I open my internet, but this time i had to log in to everyhthing. This was really weird and has never happened before. What is this? Could It be a virus? Please tell me if anyone has experienced this, and what should i do?