Just got done playing a game as Ashe. I do not know what specifically caused this bug to happen, but as I was playing, her auto attack just stopped working. She would just stand there in front of mobs/enemies and not attack, unless I was right clicking.
My items were : Phantom Dancer, Infinitys Edge, 2 Avarice Blades, Berserker Boots.
It was working fine until towards the end of the game. I don't know if this helps or not, but I was doing a lot of "orb walking" kiting some melee's with frost shot and volley. Constant running, shooting, running, shooting, in a very short time span.