I thought I'd write up what happened to me to help others with similar predicaments.
I stupidly didn't read the instructions when I updated the firmware on my a6700.
I inserted a freshly a6700 formatted SD card with the firmware update. And proceeded to start the update process. When the screen went black for a few seconds, I thought there was an error and shut off the camera, not realizing this was part of the update process. (I think it's a dumb idea to have a black screen for the update process.)
My camera wouldn't turn on. All I'd get was a black screen and a blinking red light on the bottom left side of the camera. I tried all sorts of things for hours upon hours. None of them worked.
Here's what did work:
I erased the SD card and formatted it in exFAT format on my Mac. Then I added the firmware update. Then I inserted the card and turned the camera on. After a few minutes, the screen turned on. (I think this should be done with a full battery.)
Anyways, I thought I'd have to have my camera repaired. But it turned out to be a simple solution.