It would be helpful if you would provide a little background information on the problem, such as what happened just before you noticed the problem? Was the PC moved? etc.
This problem is typically indicative of the hard drive not being detected properly, operating system/file system corruption, or even just a misconfiguration within the BIOS, but there is not any way to know until we get a bit more information.
more info please. What changed that lead to this? Was hardware/software added? Overclocking? etc etc. Please describe the events before this issue and also please list system specs so everyone may assist you better. Thanks