It's unusual for the valve body to seize up, but can happen. With a little patience they'll free up with some ATF or your favorite lube. Here's some pretty good advice lifted from HeatingHelp.
Taco Zone Valve Failure:
If it is a 570 series valve with three wires, #1 on the top, #2 in the middle and #3 on the bottom, and all terminals have a wire on them, #1 and #2 go to the thermostat and will open the valve. #2 is common to both and #2 and #3 close and start whatever is being controlled. If the valve, when opened, did not start whatever is being controlled, and you changed the valve with a power-head of known correct operation, it is highly unlikely that the power-head is bad. It is probably the control being controlled by the switch.
It is also possible that the plunger in the valve is not going down far enough and the piston in the power head isn't going down far enough.
Try this. Remove the #1 wire on the power head for a minimum of 3 minutes before twisting the power head off. This is imperative.DO NOT REMOVE THE HEAD WITHOUT FIRST DOING THIS AND DO NOT EVER TWIST THE HEAD OFF WITH POWER TO THE HEAD. SERIOUS INJURY COULD OCCUR TO YOU OR SOMEONE AROUND YOU
!
Take a large pair of water pump pliers after you have removed the head. Adjust them so one jaw is on the underside of the valve body and the other jaw is on the plunger top. Have the pliers adjusted so you can squeeze the plunger down. Squeeze the plunger down. You should feel equal resistance, all the way down. If there is rust stains and debris on the black steel plate where the plunger goes through, spray it with Kroil and squeeze it up or down. When free, re-install the head and operate the valve. If it operates and the device being controlled, works, you will need to replace the plunger and power head. If there is rust on the steel plate or in the plunger hole, you will need to replace the same above. The bellows is leaking. I buy a new valve and use the bellows assembly and power-head for replacement. The two parts as repair parts are more expensive than the whole valve. The body goes in the scrap barrel.
If the #2 and #3 wires still do not operate the control, you have a control issue. Not a valve issue.
If the above is occurring, you need to change the power-head with the valve assembly because the wax motor has probably been overloaded and stressed.
