Obtain a can of WD-40 that still has the red straw.
Hit "E-Stop" and POWER DOWN machine Lock out power
Thread the red straw from WD-40 can into groove of ball screw and into the ball nut and blow it out with copious amounts of WD-40. This will be messy, avoid being underneath the nut during this process. Take all necessary precautions to prevent eye and lung exposure to oils.
After putting a lot of WD-40 into the ball nut, attempt to spin it by hand, being careful around the belt and pulley. This will force the debris out of the nut.
Once the machine is moving again remove all the WD-40 with a rag and lubricate the machine. DO NOT leave the WD-40 residue on the ball screw. Also note that over lubricating the machine in a dusty environment frequently leads to this problem, as the dust gets stuck to the excessive debris.