First, don't panic.
Second, explain how do you unlock the bootloader, did you use this?
Third, what does the bootloader screen (Power+VolDown) says in the second to last line before the first break (LOCKED/UNLOCKED).
Fourth, if it says UNLOCKED, try to flash the ROM using this, otherwise try to unlock the bootloader again with the link i posted in the second point.
And don't worry as long as you can enter fastboot mode (bootloader) you can fix everything, and nothing of the things you are doing affects fastboot mode.
I left it plugged into my PC last night because It wouldn't turn off, and I think it wasn't charging because now it's off and won't turn on. I'm going to plug it into a wall charger and wait a few to check if it's on.
After that, I'll try the method you said.
EDIT: Seems like charging doesn't work either, weird... because it said it was connecting to my PC last night in Fastboot