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.