If you can physically move the valve then the problem is in the electronics inside the actuator. If you can work on getting that cover off you might be able to clean the contacts that operate the valve, if not, you will have to replace the valve.
When my valve died all the plastic gearing in it simply broke apart in several chucks so I had to replace it. I would ask (for future use) does you actuator sit out in the open, uncovered environment? If so you might want to consider some sort of home made cover for it (like a bucket).
Bob E.