Hi,
My immediate thought is that, when the heated bed activated, a poor ground connection between the power supply and electronics caused the USB communication to fail.
If you have a kit-built Mendel90, check the connections to the Melzi from the power supply are secure. I periodically check that all screw terminal connections are secure as part of my maintenance routine.
Regards,
Neil Darlow
I try to write with consideration for all nationalities. Please let me know if something is unclear.
Printing with Mendel90 from fedora 25 using Cura, FreeCAD, MeshLab, OpenSCAD, Skeinforge and Slic3r tools.