Broken or disengaged pawl.
The Command Dial is the true Achilles Heel of the EOS 5/A2/A2e. It is a flimsy piece of engineering. My EOS 5 has had this dial replaced twice. The culprit is in the small plastic notches under the dial and what happens if the dial is even moderately forced, the pawl ring scrapes over the notches (or conversely, locks into one) and the dial becomes a freewheel, or is difficult to move at all.

A repair job is required to replace the entire dial face and pawl assembly. It's quick and simple. The other weak link with this particular model is the lens release button — also prone to sudden failure.