I was using this specific Pi for over a month now without any issues. Today, out of the blue, the screen started flickering. I decided to shut it down. When I replugged it, the green led wasn't flashing and the red led was always on. I tried to boot with multiple SD cards (including freshly flashed ones) but It never booted. In all my efforts the red led was always on. Any thoughts?
1 Answer
First, I see in your comment that you tested the official PSU (after the accident), but what about during the accident? With the new RPi 3B+ the power supply is very important. Use only 2.5A or higher and never use PSU that charge tablets/phones/batteries (usually they decrease their voltage while load increases).
The red light according to eLinux.org means: PWR LED is also a power good indicator that goes off when power drops below 4.65V. The green should be flashing during SD card activity
Your problem sounds like a hardware failure, may be overheating. Did you measure the temperature when the problem occurred? Raspbian can show current temp on the task menu. Did you use a heatsink, good aluminum case, or fan?
While the following will not fix your problem, you can find if your RPi boots at all. You will need a USB to TTL (RS232) serial cable like Adafruit 954 or FTDI TTL-232R-RPi. I would go with FTDI, but the price is very steep. When you connect the cable to your Mac/PC you will be able to see some more info while RPi boots 
Adafruit and eLinux.org have tutorials how to use the cable.
usb to my computerwill never supply enough power. Regardless, I had an issue like this once - was a dirty connection on the SD slot - in my case it I just cleaned the connectors and all worked again