Screen says Operating System not found.

That just happened to me, and I'll tell you how I got my computer to boot up normally so I could create my backup disks.

When the computer reboots and you see the HP screen, hurry and press F10 to go to the setup screen. Go to 'Run Diagnostics' and the other tabs (I frogot which one) until you see the words 'return to system default'. Save and exit out of setup. The next time my computer boots itself the problem was fixed. I recommend taking it to a computer repair shop to see what the problem is.

Either the OS installation was damaged to where the computer cannot recognize it or the hard drive is damaged and needs replacement. You could try and use the recovery disc or operating system disc that came with the computer to try and repair or re-install the OS. If the installation fails then try taking out the hard drive and installing it in another computer to see if it is working or not. You should be able to re-partition and format the drive. If not replace the drive, If the drive tests good then it might be a hardware issue with the motherboard for the laptop.