2 months ago i got a bald patch (about a fingertip big) on my facial hair next to my chin. it happened in one day, before that, i had dense hair there.now there are only a few thin and not pigmented hairs. what happened,what can i do?

Telogen effluvium.: That description sounds like telogen effluvium. In this, hair follicles in specific locations get stunned, if you will, by some sort of stressor - physical (such as a fracture), emotional, mental, nutritional, surgical. The follicles seem to eject their hairs in these areas, creating nearly circular bald patches. Takes a really long time, but the hairs do eventually return.

Blood tests : I will see my doc ,there can be multiple reasons before I say sometimes it happens ,but certainly you need to get some blood work