If the phone won't boot at all, you don't really have many options. Try a jig & see if you can get into download mode. Also search for KingRat's 'jumpstart' method where you try and boot the phone with just the charger/without the battery.
Failing that, you could try another battery, but be aware that is unlikely to be the problem, and you'll end up with a new battery you can't use if you decide to get a new phone rather than have this one repaired.
Service centre or local mobile repair shop, most likely fix = motherboard replacement.
As to why/how it happened; who knows ? We don't have the phone in front of us & opened up in order to diagnose it. Nobody here can tell you that.
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Failure to search is the leading cause of male impotence on XDA
If you don't know how to recover if things go wrong, what are you doing flashing custom roms ?