Hi Bernd.
Thanks for the feedback. Couple of updates below and further guidance needed, if anyone you can help. Apologies for the long message!
Retrieving My Files
Just over a week ago, I took out the hard disk from the PC. As it's a SATA HDD, I was unable to install it in an older PC as it did not have a SATA connection. So, I had to purchase a SATA to USB cable with a power adapter. For anyone reading this, if your
HDD is a 3.5" HDD, you must ensure you get the cable with a power adapter, otherwise the HDD wont power up. I've heard you don't need the power adapter for 2.5" HDDs, but I cannot confirm that.
Anyway, so I was able to connect the hard disk to a laptop as an external drive, and boom! ... I was able to access my files! That was the main problem solved, being able to access my files.
Restoring the HDD / Lenovo PC
What I would like to do now is get the PC working again. Not sure if I mentioned it previously, but the Lenovo PC came with Windows 8.1 installed. When Windows 10 was released, there was a free upgrade for those on Windows 8.1, and so I upgraded to Windows
10. Not sure if this has anything to do with failing boot from the flash drive with the Windows 10 installation media that I tried previously.
Anyway, to pick up on what you suggested Bernd:
Booted with the usb recovery disk without the external windows boot drive and reconneted it after the boot. now from usb revovery drive i can access the boot drive and manged to rollback the
win1o upgrade via the recovery options.
I have managed to boot from the USB flash drive, and it displays a Windows Setup window, as per the following image:

But I am a little confused as to what to do next. What should I do / select from this window? Or should I just turn off the PC, reconnect the HDD, change the boot sequence to boot from the HDD again, and see if it will boot from the HDD?
How were you able to access the "boot drive" (is that the HDD?) and rollback the Windows 10 upgrade via the recovery options, if you were booting from the USB recovery disk?
I might need detailed steps, as I am a bit of a novice. :-)
Any further help you can provide is greatly appreciated! Many thanks!