My CNC machine has suddenly started going in only one direction in the Y axis. It was working fine for several days and then started going in a single direction when I would do a manual job.

We have ran across this issue on occasion, and usually results is there is something along your wiring that is causing a fault or is not getting good contact. We would recommend going through the wiring thoroughly and checking continuity with the y-axis wires, going from your USB or Parallel BoB to the driver.