Solved power on gives blank screen but with movable cursor

I have an HP notebook, bought with Win 8, but not used for a year. Now being used. Installed Win 8.1 no problem. Anti-virus, and a few other programs, no problem. Yesterday Classic Shell (the popular 'bring back the Start menu' 3rd party item), again no problem.
But today, after normal power off, power on produced only an IMMEDIATE black screen, with just a lonely flashing cursor. So not really a boot up. Yet the mouse could move the cursor normally. And weirdly, no keys worked -- except 4 of the function keys.

No powering off/powering on then using F2 or F8, had any effect. Nor did CTRL-ALT-DEL.

So I don't see any diagnosis path, as I am clearly kind of in Windows, but kind of not!
(hence I am typing this on my previous machine)

The only faint clue I can think of is that the last thing I recall doing before this, was to briefly connect an external hard disk. But one that was working normally just before on my other machine. And anyway, I have now disconnected it.
Or could Classic Shell be the culprit, I wonder?

Any ideas, please?

Last time that happened to me, it was because of wrong GPU drivers.

If you can produce a Power button on the screen, possibly by Alt-Shift-Del, press the Power button and select Restart while pressing the Shift key. This should bring up a Troubleshooting screen offering a number of options. Select Refresh and see what happens. Refresh can take a while, and you probably don't have to watch it all, so have a cup of coffee. It's fixed a couple of dark screens I've had lately.

Im still Kinda new to windows 8.1 myself but are you sure your not just stuck on your lock screen the screen before you type your password to log in with your Microsoft account into windows? I know I was kinda scared at first when my computer did that to me for the first time before I hit a key and it brought up a screen where you log in.

I solved this myself and was just entering the 'mark solved' flag, and therefore saw the two new messages.

1) Thank you Lemasdtre. No, nothing produced anything on the screen except that blinking cursor.

2) Thank you Atomic77. No, I hit every key under the sun, then numerous combo-key sequences, to no avail.

I now solved it simply by disconnecting the battery! I would have done that right away if it had not been for that infernal blinking cursor, which showed that the system was sort of on the edge of Windows, at least.

You learn something new every day...
Thanks again for both your efforts.

You're fortunate in having an easily accessible battery to remove. The battery in the computer I just acquired can't be removed without disassembling the computer.

I have come across the same problem and taking out my battery didn't help, after searching for hours on th internet I found that windows 8 does have another feature that replaces the F8 before booting into windows. By restarting the pc 3 consecutive times before windows boots will automatically bring up the restore menu for which you can refresh or restore etc.