For the reference, when the phone locks up for whatever reason, long power button press should always be able to get it back to life (provided that the battery isn’t fully depleted). There are two kinds of long-presses though:
- a 5 second power button press should turn it off, and then another press should turn it on again - this however assumes that the PMIC still cooperates; if it locks up in some weird state, this may fail
- a 15-20 second power button press with USB unplugged (important!) should always work and turn the phone back on. This cuts the battery power from the rest of the phone for a short moment, making sure that it fully restarts. This doesn’t work with USB plugged in, as then the power is still provided by USB even when the battery is cut out, so nothing gets restarted
Ideally, the second kind wouldn’t ever be needed - but we’re still in a state where bugs happen and the first one may fail, so it’s good to know that both kinds exist.