The handle on my outdoor hose bib won't turn at all. The faucet is currently partially on and the handle will not turn either way. How can I fix it so it will work properly?
The acme thread that allows the valve to open and shut has either galled or seized due to mineral deposits. At this point, you need to remove and replace the whole valve.
Since we don't have a picture, it's difficult to give further advice, but if it's a standard frost free, the valve is actually 8-12 inches back inside a tube on a long, easily broken valve stem and it's not easily repaired once it seizes.
Remove the handle by taking out the screw in the middle. Give the metal valve stem a few moderate raps with a hammer. Don't hit it hard or you'll break something just moderate raps. This will loosen up any deposits on the inside. Then try replacing the handle and the screw and seeing if you can turn it again.
Usually there is a screw on the handle which secures it to the valve stem - you can tighten this in order to secure the handle to the stem. If the screw threads or handle is stripped you will likely need to replace the valve.
Hopefully you have a shut-off valve inside for the faucet that you can turn off in order to stop the flow of water. If not you'd need to turn off the water supply in order to fix/replace the handle.
If you are lucky, the bib will be threaded and you can simply unscrew it and replace it. Otherwise you will need to solder a new valve in place.