Z axis suddenly dropping catastrophically!

We've suddenly had this issue three times in the last few days. Midway through a cut, the z axis suddenly drops to its limit. It's happened at different points on the same program, so we think it's an issue machine side, not in the gcode. Anyone encounter this issue before, or able to offer any suggestions as to where to start fixing it? My first action will be to deep clean the machine, check the tightness of everything etc.