Do you know exactly which processor you have? i.e. What model and speed your chip rated for.
My best guess is that your BIOS changed the bus-speed or CPU-multiplier. I'm sure those words sound foreign, so I'll briefly explain. The speed your processor runs at is produced by multiplying the speed of the bus with the CPU-multiplier, both of which are set in the BIOS. If one of the settings is reduced, the chip will run at a reduced speed. If your processor is supposed to run on a bus-speed of 166MHz with a multiplier of 11 (producing 1826MHz) and you reduced the bus-speed to 100MHz, the effective speed would end up being 1100MHz. If this is what happened with your settings, all you would need to do is load up the BIOS settings and change the bus speed to 166.
The method of loading up your BIOS configuration varies by computer but is almost always loaded by pressing a key or combination of keys when the computer begins loading. If you watch the screen when the computer first starts loading up, you should see instructions on what button to hit in order to load the BIOS configuration. Once you're in there, you will need to look around in order to find out where to modify the bus speed. It varies greatly by computer but is usually referred to as "bus speed", "memory clock speed", "processor bus speed", etc.
Before you make any modifications however, let us know exactly what chip your computer has so that you don't accidently end up frying your chip.
