Galaxy S4 Not booting up My phone (Galaxy S4 GT 19505) suddenly shut off a few days ago without me noticing it.
I tried to turn it on but it wouldn't respond to the power button. However inserting the battery in would trigger boot, but it would shut down immediately after the Samsung Galaxy screen. (I tried recovery mode too, it would shut down after on the Galaxy screen, there was a blue line on the top left indicating that it was going to recovery mode, but it shut down a second after the Galaxy screen displayed)

I found out later that it might be the cause of the power button being stuck.
However before this, I put the phone on charging despite it being off. When I took it off later and tried to reinsert the battery,it wouldn't respond at all. Now it doesn't display anything at all.

I'd like to know if something else might be the matter other than the power button, whether charging might've damaged something. Or if this is just the battery dying, I don't have a spare battery.

Thank You.
Most probably the issue is with the battery. The battery can't provide stable power output and that's why the phone can't continue after the samsung logo.
You should try a new battery. If still the issue persist, let me know.