[Galaxy S 5 Q&A, Help & Troubleshooting] insanityblink:
tl;dr at the bottom

Alright so I'm out of ideas on how to fix this phone, couple of days ago my mom came to me complaining that her S5 was giving her trouble. When she handed me the phone, it was in a bootloop of showing the Samsung Galaxy S5 screen, playing the theme and vibrating afterwards then rebooting. I took out the battery and tried to power it on, it ended up successfully booting into android. So from here, I figured there must have been some malware on the device, I downloaded Malwarebytes from the Google Play Store and ran a scan on the device. However in the middle of scanning, the device turned off it's screen and rebooted. (Malwarebytes found 2 malware on the device at that point) So now I'm convinced it's most likely malware and eventually removed all the detected malware on the device. I gave it back to my mom thinking it was fixed.

Moments later she came back saying that it's still giving issues, my first thought "wdf, it was working wasn't it?". I looked at the phone again, and tried to wake up the device using the power button. The 2 soft keys at the bottom lit up however the screen was still black. At this point, the phone was still working fine, the only issue was the screen wouldn't wake up. So I decided to do a battery pull once again to reboot it and it results in the bootloop once again. I did some googling and found various solutions to this, one of them involved wiping the cache from recovery, so it tried it. Booted into recovery and wiped the partition cache (i think that was it), after it wiped and was about to reboot, I noticed a brief "error icon" just before it rebooted. After this point I could not boot into recovery at all (it would just show a black screen and I would assume that the phone turned off) and it was still in a bootloop. It was only after trying to turn on the phone after trying to boot into recovery that the idea of the phone wasn't off and the screen was just black entered my mind. (Because usually if the phone is off, you can turn it on by holding the power button, the phone would not turn on until after a battery pull)

So at this point, it was late at night and I was tired. I left the battery out and just went to bed. The next day I tried to turn it on again and to my surprise it booted up fine but I was skeptical. I had a theory that the device would work fine as long as the screen remained on, so I set the screen timeout to 10 minutes and kept it on for probably 30 minutes while I continued to google. Since it stayed working during that period of time, I thought perhaps locking the device triggers the issue. I turned of "Power Button instantly locks device" and proceeded to turn off the screen using the power button. When I tried to turn it on, it was back to the same problem (black screen, device is working fine, soft keys lit up). I did a battery pull once again in an attempt to reboot and now it was back to problem #2 (boot loop, can't enter recovery). At this point in time, I considered doing a factory reset but since I couldn't enter recovery, I'd most likely have to re-flash the stock firmware.

Since I didn't have much experience flashing android devices (I personally only flashed 2 or 3 devices ever, all of them were successful but I still didn't really trust myself), I had it sent to a local cellphone company and had them try to fix it. Eventually I got the device back, but they were unable to fix the problem (they tried to flash a stock and custom rom). Now I'm back to square 1, the phone is still in a bootloop, no sound, no vibration this time, I don't have much experience with phones since I've only owned 2 in my life. The phone seems to be soft bricked pretty good, I've still got 1 more trick to try (full stock firmware flash) before I call it quits, any ideas?

tl;dr

- device is stuck in a bootloop (issue 1)

- i was able to enter recovery mode once, now it's not accessible

- download mode is accessible

- the device has been able to boot successfully a couple times, but then it has another issue.

- device's screen would not wake up after turning off, softkeys still worked, everything else still worked. (issue 2)

- the device was sent to a local phone repair company to fix (unsuccessful)

- currently device is stuck on issue 1

additional notes

- device has not been dropped or wet

- device has been owned for more than a year without problems

- this problem was recent (3 days)

- no sd card

- dark screen was turned off

theories

- screen could be malfunctioning or defective somehow, but then if it can show the bootscreen and download mode screen fine it seems unlikely also since it's been working fine for over a year

- it's a software issue of some sort, but has already been flashed by local phone repair to no success, then again I'm assuming they just did a basic recovery flash and called it quits