If a axis drifts (control cannot keep it in place) there is uaually a problem with the position loop. The X axis motor on this machine is on the right side of the table. The encoder cable can be damaged by chips. It also gets hardened by oil and coolant, the outside covering of the cable cracks, it flexes more at the crack and the wires break there.
The bulb in the Heidenhain encoder eventually silvers and does not count any more.
The "cable slep" flexible wire way gets damaged, parts fall off, chips fall in and damage the encoder cable.
I also have seen the axis tighten up, the motor run hot and fry the encoder.
1st thing to do is go to the maintenance page, do NOT turn the drives on, turn the ball screw by hand, and see if it counts. The ball screw should turn with one hand with a rag on it.
Note that X and Y motor are the same and interchangeable. Same for X and Y drives. Z is not.