I got it now.

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Hi,
I know this may be too late, BUT to avoid grief in the future, you should be aware that if you specify boot/root drives as /dev/sd* grub (and lilo) will look for that drive. Booting with a USB drive attached may well have your "shifted" the drive detection order so that the USB drive is /dev/sda and the drive that was /dev/sda is now /dev/sdb ... at that point all bets are off!

I don't know how Ubuntu sets up grub but you should set up grub to use UUID's in place of device names. If you are a Linux novice this may be a bit of a stretch for you but it is well worth the effort to get it right.

The following URL is a good place to start:-
https://help.ubuntu.com/community/UsingUUID

As always, of course, YMMV

cheers
pete

pete hilton
saruman@ruvolo-hilton.org

Thank you, Pete. No, it's not late, I am stuck without any progress at the moment. The number one priority is to rescue all my data first, but all I did so far I plugged in the flash drive with the Ubuntu iso on it, and it didn't trigger anything this time.
I'm super cautious to do anything with little understanding, so I rather delay it than to make another mistake.

I will try to change settings to use of UUID's, once I got my data rescued... Thank you for the URL!!

Most BIOS boot usb as an alternative to hd. It's difficult to get from sda to usb because it's a catch-22 on usb drivers.

what, you still haven't booted from a linux live medium to look at the situation "from the outside"???

I tried but the machine didn't react to having the USB drive plugged in while turning it on. I am not an expert and I don't want to do something wrong.

Then what is the solution?
Does anyone know a webpage at least, that would explain a way to do it?

It's not always possible - pot luck with the BIOS.

If not, have your usb stick inserted and press whatever key gives you a choice of boot options, usually, F9, F10, or F12. Your usb key shoulod be offered, select it and boot.