Hi, if you have loosen the screws holding the y-motor, it will not have enough force any more to move the y-axis, because the short belt will be slightly loose too (it will just slip, when the motor turns).
I too had the problem with the partly stuck y-axis (hard to move by hand), when i received my UM2 (along with some other common assembly issues), and it was getting even worse after some prints until eventually a loud squeeking noise appeared (which i could localize at the y-motor) and i was having the layer shifts in y-axis too.
I had to unmount the left metal cover and the y-motor. The