lux E won't work during Gameplay

Okay so I was supporting Lux and I tried to throw an E at the enemy Varus and it suddenly disappeared. I was like "WHAT", and I tried using it again and then I see at the bottom it says "Cast not ready" or something and then the icon for E looked like it wasn't unlocked. I thought of ranking up my E seeing if it would work but it didn't! And then I thought of leaving the game and rejoining see if that was the problem but it still didn't work. I hovered over the ability and it says "0 mana" and I was surprised. Please help and fix this please! Thank you.