If these are stock case fans the chances of it needing replacement are higher than average.
Best practice is examining the cable to make sure it hasn't been pinched or otherwise broken.
Being very careful of your fingers try to gently start the fan blade spinning from the outside by removing front panel.
If nothing yet, unscrew and unplug the fan. Unplug rear fan and plug this one in to see if starting the computer makes it spin.
Either fan or mobo is faulty.
If plugging it into pins for rear fan bring it to life your easiest options are a fan controller or molex powered adapter that runs front fan at full speed.
If fan still won't spin you should try to follow guide here for lubing or replace it.
I've seen a number of complaints recently about users experiencing problems with the fans in their system. Most fans are serviceable by design. Whether they're a case fan, a fan mounted on a CPU heatsink or a fan mounted on a GPU heatsink, the vast majority of fans can be serviced. Today, I'm...
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