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 problem was, that the motor pulley was slightly loose and wandered slowly outwards along the motor shaft, just enough that it made contact with the left panel wall and therefore was causing a lot of friction and also the squeeking noise.
After pushing the the pulley back in place (it has to be flush with the end of the motor shaft), tightening the grub screw and putting everything back together, the problem was gone. I could move both directions by hand easily and also the layer shift was gone.
When mounting the motor again, you have to push it down with one hand quite hard while tightening the mounting screws with the other. This makes sure that the short belt is tight.
Kind regards
Neni