Samsung Galaxy Note 4 — HELP! Galaxy Note 4. Screen/power off/restart issues/cryptic system forced download message!

Hello.

I have a Galaxy Note 4. It had 72% battery remaining.

When my alarm went off today, whereas normally the screen illuminates allowing one to swipe the alarm off, today the screen wouldn't activate.
I tried pressing the power button to start the screen, but no joy. Tried keeping it pressed to see if that would force the restart/power off option menu, but again no joy.
I removed the battery and replaced it. No joy.
I used a spare battery and it started up with a screen saying...

Could not do normal reboot.
ddi:mmc_read failed
QDIN MODE (HIGH SPEED)
Product name: SM-N910F
Current Binary: Samsung Official
System Status: Custom
Reactivation Lock: Off
Knox Warranty void: 0x0
Qualcom SecureBoot: Enable (CSB)
RP SWREV: S1, T1, R1, A, P1
Secure download: Enable
VDC Start

Downloading...
Do not turn off target!!

It has now been downloading for over 5 hours.

Does anyone have an idea what happened, what caused it, how long should the download take (it seem's frozen, as there is no dialogue as to progress), if there is anything I should do or look for or do & is it likely to work again?

Many thanks.

— modified on Jun 2, 2018, 8:59:32 PM