There can be two things at play here and for that there are two things you can do/try.
1. Check BIOS settings.
certain settings can be found/only be found in the BIOS, one of these settings that can be found in the BIOS are the fan settings, you can check here if there is something wrong there.
2. Sensor broke
It is possible that the sensor of the fan might have broken, i know there is away to check that with a GUI with Ubuntu and Windows so i will explain both.
To check your sensor with Ubuntu you need to have
hardinfo installed you do this by using the command
sudo apt-get install hardinfo with this you can see if the sensor is okay or broken.
To check your sensor with Windows you will need to install Speedfan which can be found here. it will show you the temperatures your fansensors are reading and the speed of your fans
(side note: it has been a while since i used these programs so they might be different then i remember them)
i hope this will solve your problem cause it did for me