Well, here is the solutions:
Solution 1: Just restart you phone and it will be normal again.
Solution 2: For this one you don't need to restart you phone.
- Click on the gear icon just above the hotspot icon, as you can see in the picture above.
- Then choose "Tethering and Mobile hotspot".
- Over here you might see the hotspot is off, just click on it.
- Now you will see a slide bar just next to "Mobile hotspot", just click on it to turn it on then might turn it off if you want to.