[GAMEPLAY] Can't use one of my summoner spells

I was played a match earlier, and could not use one of my summoner spells. I was playing as pyke, with unsealed spellbook, and I switched out my exhaust (which I believe was on cooldown) for heal (2nd spell, on F), but then I couldn't use that spell. It was greyed out, and when I tried to use it I got a message saying "you can't use that right now". After that, no matter how much I swapped that spell, and what I swapped it to, I couldn't use it. I was able to use my other spell (1st spell, on D), regardless of how much I swapped it or what I swapped it to. I don't really know what could've cause it, but I feel like this is a pretty major bug that should be investigated.