My laptop was initially running Windows 8 and I installed Ubuntu some years ago to dual boot. The dual boot was working fine (to access both Ubuntu and Windows).
As I was less and less using Windows (I dit not touch it for at least one year), I finally decided to backup all my files on the Windows partition before deleting it. But when I tried accessing it by selecting "Windows Boot Manager" in GRUB as I used to do, I got stuck on a purple screen, with no option but to shut down the laptop. Multiple attempts did not succeed. However, Ubuntu always boots normally, as usual.
boot-repair (the "recommended repair"), and it added some Windows options in GRUB (notably a Windows "bootmgfw.efi"), but all of them give the same result (purple screen, no boot).
I remember having a problem in Ubuntu several months ago (power shutdown during an Ubuntu upgrade, ended up with a broken GUI, and had to reinstall quite a bit of things), and I see this as the only possible explanation of why Windows does not boot anymore.
I know there are tens of posts dealing with hanging purple screens, and dual booting problems, such as this one, and I have read a lot of them, but they all eventually seem to derive either to some manipulations I don't understand, or to possibly risky things. I do not want to risk losing access to Ubuntu. Accessing my old files on the Windows partition (to back them up) would actually be sufficient.
Could anyone please provide a safe method (make 100% sure my current Ubuntu is not touched and works as it does) to achieve this? The ability to boot again to Windows would only be a bonus for me, since I am planning to delete it.