Two of my Teensy 3.5s (S/N 474530 & 493475, both with PID 0486), after working for several hours, are suddenly no longer recognized by Windows 7/64 as providing a USB connection, so they can't be loaded with programs. Other Teensy 3.5s here, including two with the same PID, continue to be fine. What has happened? Also, what (if anything) can be tried to revive the two dead units?