It’s possible that there’s no bootloader on the Teensy boards, go ahead and [fill in the form on this page with the URL to this forum post and your order number and we can get those back and send replacements for the 9 non functional boards.](Return Policy - SparkFun Electronics)
Hello, me again. I’ve got my Teensy MicroMod working once more. Can I suggest if yours is definitely unresponsive even after resets and you’re not able/ unable to return it, that you try blasting the large cap and bootloader chip on the top of the PCB with heat from a SMD hot air gun. This cured my problem.
Another TeensyMM has recently failed during regular use and I fixed it by reflowing the main BGA chip. I think the board is flexing and pulling the solder connections off the bottom of the large, stiff iMXRT IC. Either a flexing or soldering issue. The bootloader and cap are probably not to blame. This also explains intermittent and progressive failures (blinking codes, no USB…) that some others have had.