Fiddlesticks Unable To Cast Targeted Abilities

Dunno if this is a one-off case, or if the enemy Rengar was indeed hacking, but for the first ~11 minutes of my last match everything was fine. Come the 15 minute mark, Rengar ults onto me. I respond with my Fear and my Silence, in that order, then try to end it up with a Drain to heal back the damage from his ult before the fear ended. Except, none of that happened. I didn't hit the buttons too fast, I definitely didn't not have my mouse over him, and yet none of my abilities would cast. And it's not that they didn't affect him, like a Morgana shield or a cleanse, it's that they physically did not cast in the first place. No cooldown, no animation. Just me standing there, taking Rengar's 4 second rotation and then dying. Without doing a damn thing about it.