There are a number of ways a Raspberry Pi may appear "dead" or "bricked". Most times when a Raspberry Pi appears non-functioning there is a benign issue that may be resolved.

I, on the other hand, managed to go all out and truly break my Pi!

Symptoms:

A diagnostic video by Chicago Electronic Distributors was the only reliable test for me in the end. This revealed that my 3.3V line seemed bad. The voltage was at 0 while the 5V line seemed healthy at 5.25V.

This is most likely because I recently accidentally connected my Pi to both the red pin (fourth wire) on a USB UART serial adapter at the same time I powered the Pi over USB. This was a mistake. As Adafruit suggests;

The important thing here is to only power it from one source, the USB power adaptor or the Console Lead BUT NOT BOTH. Unless you have a Pi A+ or Pi Zero, don't connect the red wire!

After doing that for a sustained time and noticing my Pi overheating and becoming unstable I encountered the issues with my Pi not booting properly anymore. Money aside, wasting any working tech is disappointing and I have learned a valuable lesson.