mid 2015 Macbook pro won't start

There is not much you can do if dead in the water.

You can Try resetting the SMC https://support.apple.com/en-us/HT201295

Try resetting NVRAM/PRAM http://support.apple.com/kb/ht1379

In or out of warranty you can get a free over the counter 'Apple Service Diagnostics' test /assessment

Make an appointment for a "hardware issue"—

https://www.apple.com/retail/geniusbar/

One possibility is that it's in some sort of "lockup" state where it's powered on but not recoverable while power is still available. I've experienced something like that before, and the only solution was to remove the power source (i.e. the battery). The solution was to disconnect the battery, press the power button for a few seconds, then reconnect the battery. I wouldn't specifically doing it yourself on a Retina MBP (Apple doesn't consider it serviceable by anyone but Apple or an AASP), but perhaps have it evaluated and maybe mention something about it. For similar issues I've heard of the battery being disconnected or some just waited for the battery to die. The main problem was that if was difficult to figure out when that was since the battery meter (if there is one) doesn't necessarily work.

I can't guarantee that this is what's happening to you, but it's a possibility.

If you have the 15" model, then see if you qualify for the Battery Recall:

https://support.apple.com/15-inch-macbook-pro-battery-recall