A second to jviss's comments; it's a VERY critical valve. You might try taking the hose off on the outlet side of the valve and spraying a product like "PB Blaster" ( it is a great product to loosen metal parts, but takes a couple hours sometimes to work) in the opening, wait some time, and try to unstick it. Do you know what type valve it is---a Ball, a Stem Riser or Gate? Some valves will accept the Blaster better.
I'd also have a wooden plug of the appropriate size available "just in case". And of course, re-attach the hose before attempting to open it.
Hope it works----it did for me, but wasn't the raw water intake valve for me.
Let us know how things work out for you-----good luck! Jim Huhta