Samsung Galaxy S4 won't start unless connected to powersource. Why?

A

AC Question

Samsung Galaxy S4 wont start unless connected to powersource.

So last night when i went to charge my phone (it charges every night while im sleeping).

But this morning my alarm didnt ring because my phone for some reason was turned off, so i try to turn it on again (no longer charging it) and the Samsung galaxy S4 logo appears and then the phone instantly shuts down again.

Then i tried to boot it up WHILE charging, that works but it keeps saying the battery is on 0% no matter how long i let it charge, and when i remove the charger, the phone turns off instalty again.

at which point i thought it was my battery that was giving me problems. but my mom has the exact same phone so i tried to put my battery into her phone and the battery was 100% charged and working without any problems, i put it back into my phone, tried to boot it up, but then it turns off again instantly after showing me the Samsung Galaxy S4 logo just as before.

I then tried to charge it again and start the phone up while charging (which i can) but it just stays at 0 % power and turns off the second i remove the charger.

i tried to remove my SD card and boot it up, no difference.

i tried to smack the phone on the side where the Power button is.

i tried starting the phone by holding Power button AND volume down. which gives me the option to:

Volume down:cancel(restart phone)

Volume up:Continue (download a custom os or something)

and if i press restart (volume down) it continues past the samsung galaxy s4 logo screen and continues to show the fansy "samsung" animation aswell as giving me a glipmse of where i enter my pincode (for like 1-2 sec) and then shuts off again.

is this the "samsung galaxy ****ty power button stuck problem"?

or is it something else entirely?

any help would be greatly appreciated.