I understand that you have been overclocking using a OS based application, is that correct?
If yes please boot into OS using safe mode, you should be able to do that I hope. If not then remove all the additional monitor connections and just leave one connected to a PC screen.
Further remove all the HDD that you have connected and only leave the one with your OS installed.
Now in BIOS F5 and enter.
F10 and enter to save the default settings (I am assuming you are not in RAID or that you have installed your OS in IDE mode)
F8 to boot into Safe mode.
Use the control panel to remove the application you are using to overclock your system. Reset and boot into OS.
Once you are back in OS you should be good to go.
To help us help you - please provide as much information about your system and the problem as possible.