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?
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.