Galaxy S6 wasn't charging, now "Downloading, do not turn off target"

[throwthisaway]

My phone was at about 5% when I went to plug it in to charge. I left it alone for about 15 minutes, came back to see that it had dropped to 3% and the lightning bolt wasn't in the battery life icon. I figured it was just the charger I was using, and found my spare. Same thing happened until I decided to power it completely off. It was finally charging so I turned it back on, then it stopped again. Frustrated, I powered it back off and plugged the charger back in. As soon as I plugged it in, I was greeted with a teal screen with a white download icon and "Downloading... Do not turn off target" below it. In the top left corner, it says this in small font:
FACTORY MODE
PRODUCT NAME: SM-G920V
CURRENT BINARY: Samsung Official
SYSTEM STATUS: Official
REACTIVATION LOCK: OFF
Secure Download: Enabled
RP SWREV: B:4 K:3 S:3

What is happening? Am I losing all of my photos?? It has been sitting like this for half an hour and the power button doesn't work. What should I do?

 

[B. Diddy]

Welcome to Android Central! That's just Download Mode (aka Odin Mode), which is used to download a firmware file from your computer via USB. If you're not actively transferring a firmware file from a computer, you can force a shutdown by pressing and holding Power and Vol Down for about 15 seconds. If it keeps happening, though, one of your buttons might be malfunctioining (and triggering Odin Mode whenever you try to power on), or there might be something more serious going on with the motherboard.

 

[Rukbat]

And letting the battery fall to 5% regularly will mean a new battery in months instead of years, so your battery may already be on its last legs. Lithium polymer batteries are not deep-discharge batteries, so never let the battery fall lower than 40%. (50% will give you even longer life.) If the battery drops to 40% and there's no charger available, turn the phone off. (The battery isn't that expensive - under $20 - but you have to pay for labor, since it's not a user-replaceable battery.) I believe that Samsung charges $60 plus shipping. (And if you drop the battery to 5% on every charge cycle, you'll be replacing the battery 3 or 4 times a year.)