Top tip: The re-gen pedal acts like a cancel button too, just pull on it briefly; quick enough not to engage re-gen and ACC will cancel, find that it's better and faster than jabbing your thumb on the cancel button or pressing the brake pedal.
Another tip, if you let the ACC do the complete braking for you in traffic, tap slightly the accelerator and the car will resume from a complete stop, no need to hit any resume button. The ACC will completely and automatically stop your Volt if the car in front of you comes to a complete stop, and only in this case, slightly tapping the accelerator pedal will resume your ACC. The ACC unfortunately, can't read stop signs and traffic lights when you're not following any car ... You know, you can easily forget, so be attentive when ACC is on, don't rely on it too much.
If you tapped the regen paddle, or pressed the ACC button, or the brakes, you need to hit the resume button if the car is still in motion, otherwise, accelerate the car to put in motion (any speed greater than zero) and then hit resume button right away to re-activate the ACC.