Screen/power off/restart issues & cryptic download message.

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 failedQDIN MODE (HIGH SPEED)Product name: SM-N910FCurrent Binary: Samsung OfficialSystem Status: CustomReactivation Lock: OffKnox Warranty void: 0x0Qualcom SecureBoot: Enable (CSB)RP SWREV: S1, T1, R1, A, P1Secure download: EnableVDC Start
Downloading...Do not turn off target!!
It has now been downloading for over 4 hours.
Does anyone have an idea what happened, what caused it, how long should the download take, if there is anything I should do or look for or do & will it likely work again?

Many thanks.

Hi there. Did you do anything like change settings or install any app before this started happening?

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