Poppy ult screen lock [GAMEPLAY]

I just recently played a game as Tristana against an enemy poppy. When she used her ult on me my screen became locked. I tried the "y" key multiple times, opened up my in-game settings and tried mapping a secondary key to screen lock and that did not work either. I also tried a combination of those keys with"alt" and "ctrl", neither of which had any result. The problem persisted after i died, but then resolved itself upon respawn. I am not sure if this was an actual bug or if it is just my computer, but i would really appreciate it being looked into.