|The mboard will shut off in a short time if the bios detects no rpm from the CPU fan, to prevent the CPU from overheating and burning out.. |
Is this on a desktop computer, or a laptop or netbook?
If it's a a laptop or netbook, the case must be opened up and the fan be replaced.
If it's a desktop computer...
Remove the AC power to the case, open up the case by removing the left side panel as seen from the front of the case (I'm assuming it's a tower case), and examine the cpu fan, which is probably on top of a large heat sink on top of the CPU.
There must be a 3 or 4 wire fan connected to the 3 or 4 pin header for the CPU fan on the mboard. If that's not plugged in, or if it's plugged into the wrong fan header, the bios will not detect any rpm from the cpu fan header.
If the cpu fan / cpu heat sink has mung (dust, lint, etc.) on it, clean it off, but DO NOT use a vacuum cleaner to do that (they produce a tremendous amount of static electricity when running, and anything connected to them can discharge that to your components) - use canned air, or an air nozzle if you have access to an air compressor, or an artist's brush that can be used in small spaces, etc. It may be difficult to clean the top of the heat sink under the cpu fan - the most likely place to have mung on it - and the bottom side of the cpu fan blades unless you remove the fan. If you have a case fan, clean that too if it needs it.
If you remove the CPU fan, when you install it again, if it can be installed either way. make sure that you mount it so you can see the entire fan blade on top when it's installed, no non-moving central portion in the way. .
Also check for mung on the video card fan and heat sink if it has that, and the power supply's openings / fan.
With the cover still off, restore the AC power, start the computer and make sure the cpu fan spins - if it doesn't spin, if you're sure the power supply is working okay, don't use the computer until you have replaced it.
If it spins too slowly, and/or if it makes rattling or screeching noises, most likely to be noticed when the computer has cooled to room temp, has not been used for a while, and then is started up, the cpu fan's bearings are failing - the cpu is likely to overheat as a result of that if it can no longer spin it's full speed - replace it as soon as you can.