Check your BIOS Setup and see if there is a PC Health (or something like that) menu. See what it says.
Your CPU fan power cable MAY have become disconnected or loose. The fan may be jammed (when is the last time you opened you system and check for dust). Or the fan is just going bad, REPLACE IT!
Whatever, you should not be running your PC for long until you fix the problem.
Note that there is a nice freeware utility that can monitor your motherboard IF it is using Industry Standard Temp APIs, CPUID's HWMonitor (see screenshot). On my home PC it even sees the temp on my GeForce 7300 and is labeled a such. The labeling depends on the motherboard firmware, so generic labels MAY be used.
The download is the utility, NOT an installer, so you copy it to a folder of your choice then create a shortcut to it in your [Start] menu.
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