Black Screen after loading client (fixed 9/21/ 2019)
What happens: This issue started happening to me @ September 20 2019. I already fixed this but hopefully someone can use this in the future. So at random after playing league I went and restarted my client. Afterwards when it came back on the black box with a thin brass looking bar at the top. started to appear. It won't let me log in or close the window. In task manager you have to close a certain client or it will just keep reopening. (See link for image)
Fix: tl;ldr run in Windows XP (Service Pack 3) compatibility mode.
1.) locate the Riot Games folder and " LeagueClient.exe"
2.) Right click and hit "Properties" and navigate to the "Compatibility" Tab
3.) Check "Compatibility mode" and select "Windows XP (Service Pack 3)
4.) Reopen the League Client and you should be good to go.
Notes: No matter how many full clear reinstalls I did this was the only fix. I even tried reloading the GPU and CPU drivers a few times. This happened on my 570x Zen 2, RTX GPU system. I don't know if the league client has some issue with my Cuda drivers (I have some funny ones for ML) or if this is an x570 32 bit issue. Still was also really strange because even though the client was black and I couldn't interact with anything it would still update.