You need to leave it disconnected for the full 15 minutes; the CMOS power drains slowly.
When you power back on, use F2 to enter BIOS Setup. Then, use F9 to restore the BIOS configuration to defaults. Next, set the date and time. Finally, make any changes in the BIOS configuration that are absolutely necessary for your hardware configuration (i.e. clear CMOS error from log, proper boot order, SATA drive type, etc.). Make sure you save the configuration as you exit BIOS Setup.
In all this, I am presuming that you have already upgraded to the latest available BIOS. If not, you should do so now. Note the following: Updating the BIOS to Support 3rd Generation Intel Core Processors.
Hope this helps,
This message was posted on behalf of Intel Corporation
I suggest you to follow N. Scott Pearson recommendations and also to remove every component that you have plugged into your motherboard, RAM, hard drive, power supply, video card etc.
Try you hard drive in different SATA port.
If that does not work there are only a few real possibilities here...
1) Your hard drive controller has failed.
2) Your hard drive's Master Boot Record has failed or been compromised.
3) Your hard drive has physically been damaged.
4) Your power supply is no longer supplying the proper voltage to the drive.
5) Your BIOS has failed or been corrupted.
To find out what the issue is you will need to test your components in another system if it is possible, this is to find out which one is causing the problem.