[GAMEPLAY] Lux E disabled

I was playing a game as Lux, and about 10 minutes in to the game, my E was disabled. I had been maxing it first and got it up to level 5. Beneath the icon, there was the 5 gold pips showing that I had leveled it to 5. But when I hover the mouse over it, it showed a mana cost of 0, and I couldn't cast it. It stayed like this for the rest of the game. I tried exiting the game and coming back in, but it persisted. I was playing on the normal game client, in a normal draft game on Summoner's Rift.