Yeah... Sounds like the computer won't function at all at the settings you applied. Chances are nothing's damaged... YET...
GX1_Man is correct that clearing the BIOS will probably do the trick. You'll have to go through all the settings afterward. Everything will be set to it's default value.
Remove the battery. There may be jumpers close to the battery. Either two unbridged pins, or three pins with two being bridged. In the case of two pins, just short them to each other with something metal. If there are three pins, move the bridge so that the other pin is bridged with the center one... and back again. Replace the battery.
For the computer that won't start correctly... I'd recommend #1-TuffTEST, the $10.00 version. You download the software and it creates a bootable floppy disk. Use the disk to boot a problem computer and much of the hardware will be tested. (hard drive, memory, CPU, some mainboard functions, etc...). If the computer passes all the tests, something probably just happened with software and reloading Windows should fix everything. If something doesn't pass the test... you'll know what needs replacing.
Do yourself a favor and don't mess with the overclocking. Even mild overclocking usually requires a premium CPU fan to keep the temps safe. Definitely don't overclock if you have no way to keep an eye on your CPU temp. It's pretty easy to damage your memory. Particularly if it's not "premium".