Launcher goes either black or white and I can not use it

When I dobble click the Minecraft launcher icon on my desktop the launcher comes up but it is all either white or black.I cannot play minecraft but sometimes when I click the screen I log on to the minecraft website and now and then a thing comes up and says I need to get mojang from the app store.(There is no moiang app on the app shop)

{ I think it is on 1.12..2 on pc}