I'll assume you're running Windows on your computer.
This error means the computer isn't able to find a bootable drive. It can be caused by a few different things. First off, make sure you have no CDs in the drive and no USB drives attached to your computer. Try restarting. The reason for this is that if the computer ''thinks'' the USB drive or CD is bootable and if the BIOS is set to boot first from USB or CD it might not bother checking your hard drive at all, instead just giving this error.
If that doesn't work your problem is either a hardware fault (usually that would be a failing hard drive) or a corrupt operating system. I'd suggest getting a Linux LiveCD and running an extended SMART test on the hard drive. If it passes, try re-installing Windows. If it doesn't, you'll have to replace the drive.