CPU Fan Not Working?

If the pc is on and CPU fan is stopped running than what will happen?

It will:

Shut off if you are lucky - thermal shutdown.

If not and will not boot next time you try with a working fan - toast.

(an aside to all TF members - is this Facepalm Friday or what?)

You can burn up you cpu. Actually my computer will not start with fan not working.

omkar said:

If the pc is on and CPU fan is stopped running than what will happen?

It depends on your setup.

Many motherboars have BIOS settings that will make the board to issue an audible alarm if CPU fan (or any other fan) stops spinning or spins too slowly. Whether your motherboard has such feature and whether it is enabled - only you can know.

Otherwise, in many cases it might turn out that even when the dedicated CPU fan stops, case fans still provide enough airflow to sufficiently cool the CPU heatsink (often the case with a tower-type heatsink located close to a case fan). Whether it applies to you - only you can know.

Otherwise, if you let the computer run without adequate CPU cooling, the CPU will overheat. Thermal sensors in the CPU will shut it down abruptly.

After that, once the CPU cools down sufficiently, your computer will typically boot up again. Until the CPU overheats again, of course.