Please Help...I'm new to the iMac

I just purchased a G5 iMac and sometimes when I am on the web, it falls into sleep mode. I think. The screen goes all blue and everything dissappears from the desktop. I cannot get out of it, nor will it respond to any key stokes. Do I have a setting on that I am not aware of? Has anyone else had this problem? I really love my iMac, but it is only a week old and has done this twice. The only way to fix this is to pull the power plug from the wall. It just locks up. Thanks in advance for any help.

Called AppleCare and it turns out the tech installed the wrong ram when I purchased the machine. The new goods are in and everything has been working well since.

Awesome, I was just going to suggest checking that. When your machine goes to sleep the screen will turn black and the little white light will pulse on and off so that certainly wasn't the problem