I know this is a pretty old post, and you may have solved this already from the answer provided or on your own. But i was facing the same issue, and the provided answer was not the issue. I will share my experience/solution for those who may face this in the future.
The other answer tells you the track/channel as a whole is disabled (you can find this option by right clicking on black part of the track where the mute, solo, and record icons are), which they were not in my case. They were actually muted. Which was confusing to me, because you can toggle the mute icon for the entire track/channel in said black area with no changes. Instead highlight all the dimmed parts of the track that are usually a brighter color and probably different colors, then right click on the colored part of the track, and there will be an option to "unmute events". If this is the issue, you may notice a small "m" in the bottom left corner of each segment of the track before unmuting the events. It will then return to the brighter color and track parts will then play again.
Hope this helps, take care.
