Arduino M0 unrecognised as USB device

I have been programming my Arduino M0 from Windows 10 using Arduino IDE 1.8.5 for some months.
I was developing a sketch to talk to an HC-05 Bluetooth module connected to the M0 Tx>1 and Rx<0 pins which was starting to work. I could even download a new sketch via the USB port (on COM6) while the HC-05 was connected.

As I started to download a new version of my sketch and it failed because the M0 on COM6 was gone!

If I press the M0 reset button the following happens;
In Win10 Device Manager > Ports (COM & LPT);
At first this appears; 'Arduino M0 (Bootloader Mode) Native Port (COM4)'.
Then that disappears and a Windows message says; 'USB Device not recognised. The last USB device you connected to this computer malfunctioned, and Windows does not recognise it.
In Win10 Device Manager > Universal Serial Bus controllers, an error entry appears; 'Unknown USB Device (Device Descriptor Failed). In Properties > General > Device Status it says; 'Windows has stopped this device because it has reported problems. (Code 43). A request for the USB device descriptor failed.'

I have tried removing all the connections to the M0 except the USB, replacing the USB cable, other PC USB ports, turning off Bluetooth, double-clicking the M0 reset button, downloading the Blink program while the M0 briefly appears on COM4.

Do you have any suggestions as to what I could try next?