[GAMEPLAY] Pantheon bug; couldn't cast abilities or attack after using stun on Sion during his ult

I don't really have the resources to recreate the scenario to isolate the exact cause, but here is the rundown of all the events surrounding my lost ability to cast abilities or attack as Pantheon:

As a Sion was ulting towards me, I used my stun on him. Upon colliding with him, my Banshee's Veil's shield was consumed but I could no longer cast abilities or attack anything. I could still cast flash and smite, however. Reloading the client did not fix this issue.