If you are running your PC with Intel's Integrated Graphics, there is an issue with your Graphics Driver and it needs to be updated ASAP. If you are using a Graphics Card from a different manufacturer, the Graphics Driver may still need an update.
Checking to see if your Graphics Card Driver is up-to-date can solve many issues occurring with your game. Using the automatic tool from your Graphics Card Manufacturer can find and install updates to your driver.
I will provide a list of the common Graphics Card Manufacturers and their tools to update drivers:
If your drivers are up-to-date or if updating your driver does not fix your issue, you may need to run a "Full Repair" from the League Client. This will repair corrupted files that could be causing game issues.
If neither solution works, send a Support Ticket to Riot Support to get more thorough help.