1- Are the control cables from CNC controller to the AC servo driver loosed or opened?
2- The control board of the AC servo driver is broken or not?
3- Any parameter setting mistakes?
4- Check the X axis forward and backward key on the CNC controller under DIANOSE mode, and see if there is any changes?
5- The mechanical part is slipping or not?
In this case it is control board problem, change it then the CNC machine resumed.