X axis grinding noise and won't move

I've just finished building my 1500x1500 workbee. I was trying to surface the spoilboard for the first time today and it was all going fine for a while, but about 3/4 of the way through the operation it started making a horrible grinding noise and ground to a halt.

The issue seems to relate to the X axis. When I loosened the nut blocks and clamping collars and took the tension out of the screw, it moved freely and I was able to home the machine (which i guess means it's probably not wiring related?), but when I re-tensioned the screw it started grinding again and struggling to move. I've opened up the box and couldn't see any obvious disconnected wiring. Any other suggestions? I haven't lubed the machine yet, could that be the problem or should I be looking for something else? Any thoughts? Thanks