Client exits upon hitting Launch

As of today (without patching), my client has started closing itself as soon as it tries to load the login screen.

I'll describe the process:

• I run League and get to this screen, as normal. http://prntscr.com/c7dtvr
• I hit Launch.
• The launcher disappears, then the window of the login screen appears as literally just a black box.
• The login screen window disappears almost instantly.

The process is no longer running or anything. It just kills itself upon trying to load the interface for the login screen.