Need help to repair MKR1000 bootloader.

After uploading a sketch to a MKR1000 the MKR stopped responding to the USB. Though the sketch seems to be running the Arduino 1.6.12 IDE will not assign a USB 2.0 port to it and will not upload. The same port and USB cable are OK with an Arduino Zero. I tried another Windows 10 computer but no luck.

Maybe the bootloader got broken. I think it can be fixed but I don’t know the steps it takes to restore the bootloader and I don’t know where to get the proper bootloader image.

I have an ATMEL JTAGICE3 kit that can connect to the tiny 10-pin JTAG connector on the MKR1000 but have not used it yet. This kit is documented to be able to program both Atmel SAM and AVR units via Atmel Studio.

Is there a guide to restoring the Arduino MKR1000 bootloader and is the JTAGICE3 kit the right tool?

Before going the bootloader way did you try the "quick double tab reset button" method.

That should put the MKR on another COM port as a bootloader MKR.
At that point put a very simple sketch on such as BLINK (DO NOT BOOTLOAD IT !)

If that works your bootloader is probably fine.

That did it. Thank you very much Mr. Ballscrewbob!