I just went and looked at what I think is the auto-fill valve. Assuming I found the correct device, mine is an ASCO Red Hat, a brass valve with a green plastic box on top that has a couple wires attached. The brass valve has an IN and OUT side. My two suggestions: 1> replace the valve, or 2> put a manual gate valve on either side of the auto-fill valve. My plumbing setup has room on the IN side for a manual gate valve, not so much room on the OUT side. I assume it's a two-way valve - IN and OUT. You might make sure power is getting from the switch to the valve first.
http://www.ascovalve.com/Applications/Products/SolenoidValves.aspx"ASCO RedHat Solenoid Valves - The solenoid is an electromagnetic part of a valve, comprised of a coil, coretube, core and enclosure. The ASCO RedHat brand offers the worlds largest selection of 2-way, 3-way and 4-way solenoid valves, designed to handle the most demanding fluid control applications."