HP LAPTOP, I NEED HELP!!?

So, for some reason, my laptop shut down and when I tried to reboot, it said "Non-System Disk, or Disk Error..." I don't remember what else it said, so I tried re-booting again, because this happened to my desktop and worked perfect after re-booting, but I re-booted my HP laptop, and now it only shows a blinking white underscore [ _ ], and I figure the operating system isn't in, so...I tried to use a windows xp disk that I recently downloaded for like $100, and it didn't work! What do I do??