Your desktop PC has at least two fans: (1) CPU fan, (2) case fan. This error message indicates that the case fan is no longer working.
I have seen this on older HP Media Center PCs that I support, and in BOTH cases, the case fan continued to work without problems.
So, you have to first confirm whether or not the case fan is working. To do that, remove the side panel of the PC, restart in, and look to see if the case fan blades are moving.
If they are, this is a bad error message -- and you might be able to remove it. To do that, you need to boot into your BIOS setup (do this by repeatedly pressing the Esc key when booting, and when the HP utilities menu appears, select the option for startup settings). Once in the BIOS look for a screen that has the setting to disable fan monitoring. Change that setting. Save the BIOS changes. Reboot. After this, the error message should no longer display.
If they are not, then the fan has died and it needs to be replaced.
If you are uncomfortable opening the PC and messing around with the internals, then take your PC to a technician and have them do these steps.
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