The problem regards a cheap 5.1-based Android phone, manufactured by a company named Manta. A few days ago it randomly shut off on its own and it was impossible to turn it on because it got stuck on the producer's logo every single time. I tried all the usual stuff like ejecting the SIM, the memory card, wiping cache from the recovery menu - all to no avail. Suddenly, I found a way to turn it on - namely, if you try to turn it on unplugged, you may leave it for a few hours and it won't go past the producer's splash screen. Try to turn it on when plugged in the charger, though - and it'll power up within 30 seconds to the home screen and keep working with no problems whatsoever.
Normally I would think that it's caused by something battery related - but the thing is, it doesn't seem so. First, if you try to turn it on unplugged and leave it on that never ending splash screen, it'll hold its charge alright, not turn off after a minute. And more importantly, second - if you plug it in, turn it on and unplug it - it'll keep working perfectly until you have to turn it off or restart it. What may be the problem, then? Is there anything I may try to do to fix the issue and allow it to turn on when unplugged as well?