Ultimate stops working mid game (Vel Koz)

I don't recall the matchup exactly but I was roughly level 7-9 and had used my ult twice (and died the second time during it) after my ult was off cooldown in no longer would work. I tried clicking on the symbol, tried hitting R (which I verified still worked in chat) and nothing. We lost the game obviously but I can't play vel knowing this might happen again.