Assigning Metarial and Face getting hide

Blender error.
there is a glitch When I am trying to assign a new material in a particular face. the face is getting hidden. and I can not get that back. please help me. how can I fix that?

Hello and welcome!

Have you a screen or better a video what is going on to see your actions?

P.S. it’s not a good place to ask for help, here mostly dev and python things. A good place is and in support branch you can choise category for ask for help.