From the way you describe it, it sounds like it's a regular gate valve. You might be able to repair it as you describe by removing the packing nut and replacing whatever is broken or stripped out inside.
The only exception to that would be a ball type valve, but those will just have a handle that makes only a 1/4 turn. Those generally aren't repairable, but then again they almost never fail either.
Personally, I would just remove and replace the gate valve with a good ball valve. Gate valves have a tendency to rot out or seize up from lack of use. I actually had one once that disintegrated in my hand when I tried to turn it off (spraying water everywhere and making a nice mess). Ball valves are just much more reliable.
I'm going to assume it's a typical gate valve, and also since you said main water valve I'll assume it's the main shutoff just before the meter.
With these assumptions in mind, I'm going to say you will have to call your local municipality. The water company does not allow (or like) home owners to tamper with anything near (and defiantly not before) the water meter.
In fact, in lots of areas the municipality actually owns the pipe (and any fixtures) before the water meter. So this is not likely a DIY job.
When you call to have them come fix the valve, ask them to replace the broken gate valve with a ball valve.
If I have assumed incorrectly and the valve is on your side of the water meter, you can try to repair it yourself (see this question for how to do it) or you could just replace the broken valve.
If you are going to fix the valve yourself (it's on your side of the meter) and there is no other shut off before this valve, you will have to make a call to the municipality to have the water shut off to the building.
If you do the work yourself, you may have to get a permit and/or have your work inspected by a licensed plumber.
Best Answer
If you can't manually operate the handle, the actuator might not be at fault - the actual valve might be jammed (or the actuator might be jammed, keeping you from operating the valve by hand, I guess.)
In any case, you should probably take the actuator off and verify if the valve can be operated by hand without it in place - if not, the new one won't have much joy either.