A ball valve is frozen when it won’t close. I installed a water bypass on my pool vacuum system last year and it worked swell until startup this spring. I thought I was very thorough winterizing last fall only to find the valve blew.
The neat thing about ball valves is that they operate on a quarter of a turn and usually are watertight for hundreds of uses. The ball in a ball valve has a port through the center that allows water to flow when it is inline with the flow of water and stops the flow when it is turned sideways to water flow.
Your problem and that of many homeowners in the spring is to find a leaky valve. The cause is water trapped in a ball valve freezes when it is turned off and exposed to freezing weather.
Now when I winterize anything that has ball valves I am sure to reopen all the valves after blowing out a system to be sure they are empty.
Some garden hose adapters that have shutoffs are ball valves and left out all winter are susceptible to the same problem.
One last note of unusual winter plumbing problems is the frost-free sill-cock. More and more homes are getting them because of the convenience of having water available outside all winter and not having to shut them off in the fall. They work by having a long body that extends into the house and shutting off the water there. When the water is off, the valve drains and all is freeze-proof, unless someone leaves a hose attached so the valve cannot drain.