As far as plumbing valves go, I really dislike gate valves. They are very common however, because they are a few dollars less than a superior ball valve. Gate valves have a round handle that, as you turn it, lifts or lowers a gate that either stops the flow of water, or allows it to flow, depending upon which way you turn the handle.
The problem is that the handle will shear away from the gate which makes the valve useless. The gate may stay in the up position (allowing water to flow), or it may drop down and stop the flow of water (big problem if you need water). The answer is to replace the gate valve with a ball valve. However, you can restore the flow of water without replacing the valve.
Use opposing wrenches and remove the top of the valve. You should hold the bottom half of the valve still with one wrench, while applying the other wrench to the top half of the valve. Unscrew the top of the valve and remove it. Reach down inside the valve body and remove the broken gate (it will look like a piece of rounded metal sitting in a groove). You can screw the top back on to the valve and turn the water back on. This will allow you to have running water, but you won’t be able to shut it off from this valve. The best choice is to replace the gate valve with a ball valve.Install Trim Around Attic Ladder Opening » « Kitchen Faucet Hot And Cold Water Is Reversed