Try removing the battery and SIM card, wait a minute, put everything back together and immediately connect the phone to the power supply. It should turn back on. Try doing this with different chargers, maybe even with a slow charger like a PC.
The same thing happened to me before I got to my problem: I broke the screen, I didn’t fix it right away and I started having problems with the power button. After that I fixed the screen, but the power button continued to give me problems, until I ended up in a bigger problem (see here FP4 stuck in Recovery Mode + broken power button = can't proceed). I hope you don’t go down the same path, but in the meantime try to see if you can turn on the phone and access the app.