Under the handles there is a nut. This is a packing nut and may have been over tightened. Loosen it a bit and see if the handles will turn. You can do this without turning off the water supply. If you need to replace them, then the water needs to be turned off. Since these valves do not get operated often, they can stick and may have corrosion built up around the gate.
If you have never made a mistake, you haven't done much.
Last edited by majakdragon; 11-01-2009 at 10:39 AM.
Reason: additional info