Xiaomi Redmi 4 Prime does not boot anymore. Stuck on MI Logo. Please help recover - Android Q&A, Help & Troubleshooting

Hello all together,
i´ve got an Xiaomi Redmi 4 Prime (3G/32GB) FullHD Smartphone which i was very happy the last 3 years. It worked smooth and i had a great experience. Every evening when i go to bed, i shut down the phone and reboot it in the morning. So i did one week before. But on next morning phone does not boot any more. It was stuck on MI Logo.
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I could not do ANYTHING.
Could not even remove battery because it is not removable. All i could do is to wait until fully charged battery did go down to 0%.
So i tried rebooting many time without success. Looking at the internet i found some users reporting same problem, but most did have problems after an update or flashing or something. My Phone is Stock, i ordered it from china and never flashed anything. I did have Google Play Store and good working OS (Android 6.0 and MIUI 8). In the phone itself i often tried to look for updates but all time it told me i am up to date. i didn´t care about having an old OS. It was working and ok for me.
Ok so i did do alot of searches in google and found some instructions like how to boot into recovery mode or how to boot into fastboot mode.
As a software developer, i already had Android Studio on my machine and i did try to use the platform-tools from the SDK (adb.exe and fastboot.exe)
After some drivers update i can tell if iam in recovery mode, adb is listing my device
so when i type "adb devices" i get
List of devices attached
e0991c307d03 sideload
Click to expand...
Click to collapse
i did try a "adb pull /sdcard" but i get error: connection failed: closed
so i tried with fastboot
i found some information like booting twrp like a live cd with fastboot boot twrp.img (i did download a twrp version from the internet)
but i had no success and got that message
downloading boot.img OKAY [0.119s]
booting FAILED (Status read failed(Too many links))
Finished. Total time:
Click to expand...
Click to collapse
i tried fastboot erase cache, this seem to be worked, but my phone does not boot
fastboot oem device-info is giving me this info
(bootloader) Device tampered: false
(bootloader) Device unlocked: false
(bootloader) Device critical unlocked: false
(bootloader) Charger screen enabled: true
(bootloader) Display panel:
OKAY...
Click to expand...
Click to collapse
fastboot getvar all
(bootloader) version: 0.5
(bootloader) battery-soc-ok:yes
(bootloader) variant: QRD eMMC
(bootloader) secure: yes
(bootloader) version-baseband:
(bootloader) version-bootloader:
(bootloader) display-panel:
(bootloader) off-mode-charge:1
Click to expand...
Click to collapse
So now WHAT I WANT!!!
I want to boot my phone to like it was WITHOUT loosing ANY DATA.
or at LEAST, recover the FILES from the internal 32GB storage.
I have WhatsApp Conversation and Media Files from near 4 years which i don´t want to miss.
I´ll be thankfully for every help i get from you. Please, before that trouble i have now, i never used adb, fastboot, twrp or anything like that before.
But why does it not boot any more, i just powered it off
Maybe the internal storage is near full, that is the only reason i could think. so erasing some files might help
For this guy who can help me to reboot my phone without loosing any data in it i will even donate some bucks via paypal to you.:fingers-crossed:

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did you try to just press the Power down for 10-15 seconds to force the Redmi 4 to reboot back into Android manually?

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Hi, pressing and holding the power button for more than 10 seconds simply does reboot the phone, but it is still locked on the boot screen (MI Logo)
I did now try and read many things.
I am successfully able to enter fastboot mode, but i see no fastboot command to access (and recover) any files from the internal storage.
i WAS able that the devices was listed unter adb devices in sideload mode?? but unfortunatelly in sideload mode, the adb pull command seems not to work?
there seems to be other adb modes besides adb sideload mode??
3. Mode i can enter is in download mode, i had to install some qualcomm drivers and the QPST. It contains a lot of tools and it seems to offer also a backup software, but i does not seem to work. also i found a EFS Explorer which also does not seem to work.
IT MUST be possible to at LEAST READ and BACKUP any files from the internal storage. I don´t care if the phone will remain dead, but at least i want to access and read the internal files. It can not be possible that so many members do not have any solution, even if i offer some money on a successful solution