Xearth "Q" ability bug

Xearth's Q stops working completely. I tried clicking on it, changing the key, nothing worked. it stopped working and restarted the next match I played with him. This has happened to me twice in about a 3-5 month period. It is rare, but it hurts when it happens. It makes Xearth a really bad champ only having 3 abilities. Riot, please fix this bug!