Your problem may be as simple as a small stone in the valve and all you need to do is clean it. If you take the top off, be very careful as there as small parts that can be lost in the blink of an eye. Since you appear to be a real novice with these things, it might be worth your while to have a professional handle this. If more than cleaning is needed, a pro can have everything back to top condition very quickly. Often valves are solvent welded into the manifold so it really is a tricky DIY job, especially with no knowledge of what you are working on. I'm a little curious as to why your irrigation water is still on in December.