Sorry that my first post is a negative one. I'll tell my story here as a last resort before I return the unit.
I’ve checked all the points in the Boot Problems Sticky. This is my first Pi, but I have experience with windows, mac, linux and Arduino so that you know this isn’t my first rodeo.
The Good News:
I got this 3B+ from Fry’s Electronics using Google Express for $35. I had no previous experience with Raspberry to this point, however I put a no-name 8GB mini-sd formatted using the suggested sdcard.org “SD Memory Card Formatter,” copied NOOBS to the root, downloaded and installed raspbian without any problems. Raspbian updated twice, and all seemed fine, everything was working perfectly!
The Bad News:
Then suddenly the screen went black, and I couldn’t get any response from the keyboard or mouse. I tried rebooting the pi, but it showed no sign of life.
I was relieved when I read on the forums that SD cards can go bad instantly. I was sure this must be my problem! After all I had used a crappy SD card, and had performed a ton of I/O on it that afternoon. I tried another SD card but still nothing.
So (I knew I would have to eventually) I ordered a genuine Samsung 64GB MicroSDXC EVO Plus Memory Card from amazon. When it arrived I was sure all would be well, however I still couldn’t get even a single flash from the activity LED no matter which technique I attempted to format & boot (sdcard.org + NOOBS / ETCHER + IMG.) Incidentally the Power LED has been steady and solid, I’m using a good quality 2.4A Power supply.
Let me know if you can think of anything else to try. It’s frustrating that the activity LED won’t give me any indication of what’s going on, and there is no onboard bios I can interrogate. The bright side is I’ve learned tons about how the Pi boots up, and that if it can’t, you’re SOL