OK, I Turned Up the Clock and Now it Won’t Boot Up!
That is ok, you just turned it up too far. If your system gets far enough along in the boot up that you can get back into your BIOS, do so. If your system won’t boot up at all, you will need to reset the CMOS. How to do this is dependent on your motherboard, check its documentation. Usually you have to move a jumper from one place to another, turn the computer on, then turn it off, put the jumper back to the original location, and turn it on again. Some motherboards designed to be friendly to overclockers will let you clear the CMOS with just the push of a button.
Long ago, before buttons or even jumpers were available, the only option was removing, waiting a minute, and replacing the motherboard’s battery (they use the same batteries as calculators). This is a less reliable procedure than switching a jumper, let alone pushing a button. You are supposed to be able to change jumpers all the time, but a motherboard will usually see its entire life under one owner without a battery change. Start popping the battery in and out and you risk breaking the clips that hold it in place. Though the battery is big and shiny and easy to find, don’t get lazy and use it instead of the jumpers to clear your CMOS.
Clearing the CMOS sets the BIOS back to its factory default settings. Because of this, and because it can be hard or impossible to check something online when you are having system trouble, write EVERYTHING down while overclocking. And though screwing with your CPU clock is unlikely to burn a hard drive, you really should backup your data before you change anything in the BIOS, overclocking or otherwise.
However you get back in the bios, you’ve gone as far as you can for now with your overclock, and you either need to back off on the clock, or, if you are comfortable with the risk/reward potential outlined below, increase one or more voltages.