can you spray WD-40 on it so you can turn it off?
Shouldn't be toxic. But I'd try just working the valve back and forth to see if I could loosen it.
What kind of valve? Water? Gas? If it's a gas valve, LEAVE IT ALONE and call the gas company or a plumber. If it's water, if it breaks it won't kill you, but it can make a huge and expensive mess. First, are you sure that you're turning the valve in the right direction? When facing the valve, turn it clockwise to turn it off. If that's not the problem, then using WD-40 or other petroleum based product might work, but you have to be patient with it. Let it soak in and give it time to work dissolving and loosening rust or general grunge that might be freezing the valve. There are other products developed specifically to loosen nuts and bolts, but if you don't have easy access to an auto parts store, then try this. Spray the valve generously, wait 30 minutes, work the valve gently back and forth, spray again wait 30 minutes, work it again, etc. until the valve loosens. Don't force the valve or you'll snap it.
this is something to try, but if it doesn't work, you are going to need to call in a pro for help.
