To check, screw the case shut without the Pi inside. If it closes nicely, try again with the board inside (safe to say you wouldn't want to plug it in at this point). It it doesn't close properly, it means the board is in the way, and the case is not meant to go with your Pi. Don't touch the screws, send the case back, get another one. Don't try to adjust the case, or the board to fit. Who knows what you'll end up damaging. In my case it was most likely the CPU (it's got a higher profile than model 3).
Also PoE pins could be touching the metal case (it wasn't my case... ), or other pins, plenty of things to go wrong.
I went through the sticky on boot issues, and couldn't fix the problem.
I bought a 3B+ a few days ago and wrote an image of RetroPi onto a 32 GB card. I connected a wireless keyboard dongle (logitech unifying receiver), a power supply (OnePlus 6 Charger rated at 5V, 4A), and a HDMI cable to a Samsung TV).
Worth mentioning is that the unit was enclosed inside a metal case with bits touching the lan, ram, and cpu chips, and no other electrical components. Thermal compound was used in a conservative manner, all very neatly applied, no spills.
It powered up and ran, went through some updates, powered it down (via software, not uplugging), then it froze while shutting down. I went CTRL ALT DEL on it (windows guy). It powered off, and nothing ever since.
Currently the red led stays on, and no green light whatsoever. No flash, no blink, not even briefly turning on.
What I've tried:
*Note when I say "format" I mean using the "clean" command in CMD
1. Assumed it was the corrupt data on the SD card. I've burnt the same image all over again to bring it back to the previous working state. Did not work. Tried the most recent version of Noobs (files inside the root of the formated FAT32 partition), burnt image of newest Raspbian (on card, not a partition on the card)after formatting, burnt pwnpi, then Noobs lite (again files in root), then Raspbian lite image burn onto the card (not the partition).
None of them workied.
2. Bought a couple more SD cards, thinking something went wrong on the hardware side of the SD card preventing it from being used as a booting deivce (grasping at straws). Repeated step 1 with both of them, no cookie.
3. Tested voltage with the multimetre, It gets 5.21V before the polyfuse, 5.19V after it. Tested the power adapter white unplugged from the Pi, supplying 5.21V slight variation towards 5.22V.
4. Removed the polyfuse, replaced with 0ohm rez. Red light on just like before, only now I get 5.20V after the 0ohm. Tried booting, didn't work. Soldered the polyfuse back.
5. Disconnected micro usb power, then fed 5.2V straight into the GPIO pin +2 , grounding it with pin -6. Red light on, no boot.
6. Checked the contact of the microSD card onto the board. Desoldered the metal top of the SD slot, to reveal all the pins were in good condition, gave them a slight upwards bend with a needle just in case. The presence spring made proper contact with the other metal bit, so the card could be detected. Closed back up, red light, no boot.
7. Tried powering on with only SD card and LAN cable connected, to potentially SSH into it, in case the error was caused by the hdmi cable or keyboard dongle being connected, nothing. Red light on the Pi, no green light on it, none on the RJ45 slot.
8. Went to sleep crying, woke up, red light, no boot.
9. Left it alone another day, plugged in, red light, no boot.
It think this covers pretty much everything.
Any other ideas?