Ubuntu :: Screen Blanks - Computer Still On ?

We have a computer, was windows, we installed ubuntu. It worked well for several months, but once in a while the screen would blank out. The computer was still on, the lights on the computer box are still on, but I can't get the screen to show anything. In the last couple of days, it now does this a couple of times a day. I have to hit the computer off switch, do a shut down, and reboot.

Any particular set of keys I can switch to get the screen to go back on? Like ctrl alt delete?

Also, how do I check the computer to see if there are any problems with the computer? There is system, system testing, does that check the hard drive, start up sequence, anything else? What else can I use to test the computer, system.