Some champions seem to lose their ability to autoattack. Of the ones I've noticed:
>Ashe instantly loses it the second she attacks.
>Blitzcrank loses it if he uses Power Fist and lets the buff expire without using it.
>Tryndamere lost it early but I wasn't paying attention to what caused it. Probably the same as Ashe, and if not I was able to use his E once before it stopped.
There's probably more. I've been testing every champion one by one when I have the chance.