If the phone won't boot at all, no download/recovery & not detected when you connect to PC, it's cactus, I'm afraid. Having said that, search for KingRat's 'jumpstart' method where you can try to get the phone going with just the charger/minus the battery. Unlikely to work, but you might as well try it.
Why did it happen ? Impossible to say. You were running custom firmware, so it isn't the brickbug. Many people seemed to curiously begin having hardware issues (dead WiFi, dead USB ports, dead headphone jacks, damage caused by moisture exposure taking a long time to start having effects) with this phone as it approached/went past the two year mark (I was one of these people as well).
I guess you could try a new battery & replace the USB board/port which is relatively cheap to do, but that could still leave you with a non-working phone & money spent that you didn't need to spend. More likely you need to replace the motherboard; a Samsung service centre will charge you a fair bit (mostly for labour) to put a new board in, you can source a 2nd hand board more cheaply online (Google search/eBay/Swappa) & either replace it yourself, or pay a local mobile repair shop to do the swap.
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 !
Probably the best thread ever posted on XDA
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 ?