If the propeller of a fan is not running, it could be due to various reasons. One possible reason could be a loose or disconnected power cord that connects the fan to the electrical outlet. Another reason could be a malfunctioning or faulty motor that drives the propeller. It is also possible that the propeller is stuck due to accumulated dirt or debris, or the bearings of the fan may have worn out, which causes the fan to have difficulty turning. To fix the issue, one can try unplugging and plugging the fan back in, checking and tightening any loose connections, clearing any obstructions around the blade, or lubricating the bearings to allow smooth movement. If none of these solutions work, it may be time to replace the fan.