Teensy 3.2 Died Mysteriously

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I've had a Teensy 3.2 installed in a plastic box on a custom WizNet W5500 Ethernet shield for a few weeks. The Teensy was used to read four analog inputs and send messages via Ethernet through the W5500, and was working fine until a few days ago, when it stopped responding over ethernet. I've desoldered the Teensy from the ethernet shield, and it appears that even with nothing connected to it, I can't program it from my computer (it never shows up under COM ports). I can hear some windows connect/disconnect sounds when I click the program button, and the Teensyduino app window shows "reboot ok", but I have so far been unable to reflash the Teensy from the Arduino IDE (COM port not found error).

To check whether the MKL02 chip was alive, I performed a test mentioned in this thread, where I measured the RST pad on the underside of the teensy and pressed the program button. Upon clicking the button, the RST pad goes from 3.3V to 0V, which suggests that the MLK02 chip is alive and watching its program line.

I have also confirmed that 5V appears on the 5V pad (from USB), and that 3.3V appears on the 3.3V pad (from the LDO regulator).

Does anyone know what might be causing these symptoms? My best guess so far is that I may have cooked something on the board since the plastic box it was in didn't have vent holes, and got a little bit toasty...but it would be nice to know how such a failure would occur, and what parts of the board are vulnerable to it.

Any advice would be greatly appreciated! I have a number of these Teensy 3.2 boards, and would be happy to rework SMD parts as necessary if anyone has theories to test involving swapping the main MCU, crystal, USB bridge, etc.

I have attached a picture of the first page of my schematic showing how the Teensy 3.2 was attached to the custom ethernet shield. All signals from the WizNet W5500 are 0-3.3V.

Best,
-John
 

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