Shut down your PC. Press the power button immediately press F2 to enter into BIOS setup. Press F9 ( or load the default button shown on the BIOS screen) to reset your BIOS. Press Esc or F10 and select "save and exit" press Enter and allow your system to restart then check if the fan now works.
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Check with a hardware technicians, that the wire is disconnected or any loose contact. If able to repair it or not. Otherwise replace the fan with new one.
To fix a computer fan that is not working, you can check the power supply, clean the fan, replace the fan, or update the BIOS settings. If the issue persists, it may be a hardware problem and require professional assistance.