MacBook Pro not turning on after not being used
Reset your SMC. It controls the power management hardware on your system. It may not be allowing your battery to charge for a number of reasons.
If this still happens afterwards, please take a (preferably high frame rate) video of you powering on your machine, including both the keyboard and display, so that we can see at precisely which point in the boot it fails.
EDIT:
In our discussion in comments, we ruled out possible low-hanging fruit:

Ensuring the correct key chord for SMC reset (ShiftControlOption on the left-hand side of the keyboard for pre-T2 units, + Power)
Swapping the charger cable.
Swapping the charger.
Trying all four USB-C ports.
Verifying live AC power.

Because of how early in the boot flow the failure occurs, it is likely to be due to a hardware issue. There’s very little you can do to debug this without poking at the hardware, whether it’s you or an Apple support engineer doing it. You would need to start probing voltages along the power rails and looking at power sequencing on an oscilloscope. Maybe it’s something as simple as a bad battery confusing SMC, in which case you could try to replace the battery yourself, but don’t buy one unless you’ve confirmed with a multimeter that yours is bad.
Short answer: Talk to Apple for options.