Game Freezes, Can't Close Window

I've never had an issue with client performance, but the game froze on me in _3 different games_ today. Alt + F4 didn't work, I wasn't able to pull up the task manager, and I ultimately had to restart my computer entirely in all 3 instances. Luckily one of my DC's happened seconds before the enemy team surrendered. The other two were losses with leaverbuster punishments. Worst part of all this is that I've been playing the past few days as a last ditch effort to get to Gold for the first time ever, and I'm 2 wins away from completing my promos. Anyone have any ideas? I don't want this issue to happen again as I _have_ to win 2 out of my next 3 games to finally make it to gold.
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