Why won't my Galaxy S6 turn on and randomly turn off?

[AC Question]

I bought my S6 about 3 weeks ago. I was using it normally, but then I left it and when I came back it wouldn't turn on unless I pressed the volume down key too. After that it kept turning off after 15-30 minutes I turned it on, everytime, and then it simply wouldn't turn on whatever i did, even if a pressed all 4 buttons for 10 seconds. Then I simply pressed the power button and it turned on. It behaved normally for that whole day. I went to sleep with 45% battery and it died. Again it wouldn't turn on unmattering what i did, and after like 5 hours i pressed the volume down and power again and it turned on (after many tries), wtih 27% battery - and it was always off, but after like 2 minutes it died again, and only turned on, after many tries, when I presed vlm dwn, power and home together, giving me the restart screen with 7% charge. (not that dos/cmd-like, with hard reset and all, only a custom os and restart). it worked normally (draining battery fast), and then shut down again, even if i was charging it, though the whatsapp green led is flashing, so i think it's not completely off.
What's my problem? i'm really afraid because i have really important things in it

 

[B. Diddy]

Welcome to Android Central! When you say it turns off, do you mean the screen goes to sleep, or the phone actually powers down completely?

Did you buy the phone new or used? If it's new, it shouldn't be showing anything about a custom OS. Try booting into Recovery and wiping the cache partition: https://support.t-mobile.com/docs/DOC-21095

Ultimately, you may need to bring it back to the store and ask for an exchange, or contact Samsung for warranty repair/replacement.

 

[guisvv]

Hello, just registered now, thanks for your help.
It completely powers off, since I checked whataspp web and it wasn't working. the phone is new. i'll try doind that and see what happens.
edit: only sometimes, when i press volume down along with power and home, it displays that install custom os or power normally. i've whiped cache out and so far the phone works normally, let's see.

 

[B. Diddy]

when i press volume down along with power and home, it displays that install custom os or power normally.

I think you're booting into Download Mode, which is a normal feature of Samsung phones. This is how you can download a firmware file from your computer for installation on the phone. You have to press Power, Home, and Volume Down to boot into this mode.