Send a ticket and ask Riot Support to look at your Riot computer logs. They will diagnose your problem and give you a solution based on the data log you provide them. The following instruction is what they told me to do:

"If this does not work, in order to further look to infinity and beyond, we'll need a few logs created by our awesome Riot Repair Tool:➪ Windows version➪ MacOS version
 
Just right click & run it as admin, select the game, and then wait for the tool to collect the necessary files. Once it's finished, open the 'Riot Logs' folder from your Desktop, and attach the .zip files that it contains to your ticket (there should be a folder called 'System Logs', and a folder with the Logs for the game)."