I've been using a Teensy 3.6 in my "radio" (see https://forum.pjrc.com/threads/55551...uot-MP3-player ) almost daily for over a year. Earlier this week it wouldn't turn on. I checked the power supply and 5v is properly fed to the Teensy. I measure 3.3v from the on-board regulator, so the fuse wasn't blown. I attached a PC to the USB connector and the PC doesn't see anything, like the cable isn't plugged in. I check the cable, it's fine. I hold the button down on the teensy for at least 15 seconds, and the PC sees a new Comm port when I release the button.

I reload my sketch and everything is back to normal.

What would cause the teensy to loose its programming? Some strange voltage transient?