Plumbers often recommend that homeowners locate their water main valve so they know how to turn it off in an emergency, such as if there is a major water leak. It's actually a good idea to go one step further than just locating the water main valve. You should also try to turn it. If it gets stuck, that is something to address before you're in an emergency. Here's what you should do about a stuck water main valve.
Apply Some Penetrating Oil
The first thing to try is applying some penetrating oil to the valve. You can buy penetrating oil at most hardware stores. Follow the instructions on the container to apply it to the water main valve. Typically, the instructions will tell you to apply the product liberally, wait a certain period of time (often five or ten minutes) and then turn the valve back and forth to work the oil into the valve.
If one application of penetrating oil does not do the trick, you can try a second one. If that does not work, then your best bet is to call a plumber.
Call the Plumber
A plumber might try a few different approaches to loosen and mobilize your water main valve. First, they might try a stronger penetrating oil. They sometimes have access to products you can't buy in the store that do a better job of eating through thick mineral buildup.
If that does not work, then the plumber will likely replace your main water valve. To do this, they will start by turning off your water supply. Then, they will cut out the piece of pipe to which the valve is attached. They'll replace that section of pipe, along with the valve. Then, they'll turn the water back on.
Once you have a new water main valve, you'll want to take care of it so you can keep using it as needed. Start by giving it a turn on and off every month or so. Once a year or so, you can apply a penetrating oil or lubricant to keep it more supple. If you ever notice the valve tightening again, give it another coat of penetrating oil.
Your water main valve must be able to turn on and off in order to protect your home. If it does not turn easily, this is a problem you need to address. Don't hesitate to contact a residential plumber.