Ganplank Ulti not Showing up on Cooldown

I was playing Gangplank top lane with my friends this past weekend and I used my ulti to help on mid lane. During the process of the casting I was killed by their top-laner. When I respawned it showed my ultimate being able to be used but I could not cast it. This only happened to me in that game so I am curious as to if it has any relation to being killed while casting.