Is it a stopcock or a gatevolve ? The reason I ask is because if it is a stopcock, when you replace it there is an arrow on the side showing the direction of the flow ! Which ever the best thing to do is replace it !
This is called sticktion or cold welding coupled with chemical depositation. .Must values are left in the open position. You should never, never open a value or spindle value fully, as the spindle threads will cold weld onto the body threads and the water partickles over time will also bind over the threads. Being fully wound open, there is only one ditrection of travel, that is to attempt to close the value, often impossible.
Turn off at mains, and when you have placed you new value in position, fully open it, and then turn it about 1/8 revolution to close it. You have then removed all pressure off the thread, and given yourself the option of turning the value both ways when next turning it closed.