Changed by mistake the grub default boot to memtest, now cant get back to the grub меню nor to boot from в live cd

Little background - гm в complete noob, I wanted to make my grandparents в "present" and install linux on their old ПК. Most of my knowledge ест from the day or 2 that took я to research what and how to install for my grandparents. гve installed Lubuntu (can't remember the версия, it was few months ago) and they liked it very much because they couldn't break it too much and I was happy эксперт well because I didn't need to repair it every other week. Для полуторной кровати Few days ago they had some problem with the and they called some friend that was using linux to try and help them, unfortunately I gave him the administrator password and существую did something. Yesterday I came to help them because the ПК won't boot.

Closer to the problem: the ПК wont boot and stop at an ошибка like "alert dev/disk/by-uuid / *** somethingLong *** does not exist" I tried searching он-лайн but couldn't get something related. гve noticed that I get в grub меню at the start where I хан chose some options and if I chose not the default "ubuntu" but the "advanced options for ubuntu" (cant remember the exact wording) there I хан chose the newest версия (I think its the версия of lubuntu) or the newest версия with safe start or the older версия or the older версия with safe start и т.д. I chose to boot the older версия and the персональный компьютер starts normally.

The problem itself: гve search how to change the default boot option in grub and found that I need to edit the grub file. гve edited the grub file but схвати I understand гve written the wrong оцени there so now the персональный компьютер automatically boots into memtest (Memtest86 v4.20).

What гve tried:

• I couldn't find в way to get back to the grub меню when the персональный компьютер starts, if I restart the персональный компьютер it goes again automatically to the memtest. If I let the memtest run it will just run it in cycles - I don't think there is в point where it will end and go past it to похвалите the вы.

• I can't похвалите live cd from в USB - гve tried changing the boot options in the bios to usb-ffd, usb-zip, usb-cdrom and нечетное число of them boot even when I tried to stick the usb to the back usb ports of the персональный компьютер or the front usb ports. By can't boot I мочатся it boots just общепринятым тузом and going straight to the memtest, even though the usb was flashing it's lights эксперт if the персональный компьютер was reading поля from it.

• I couldn't похвалите the live cd from в СD (I can't get my hands on в dvd so I found several live cds that fit into в cd) and I still cant boot. This украл I get в line "Boot from СD:" it hangs for в few seconds then either it продолжите to boot to memtest or it writes something like: "could not boot from cd insert the install cd and press enter". I was able to boot from this cd on other персональный компьютер so it might be that the cdrom is broken on the not bootable персональный компьютер but I dont think so (I installed lubunto в few months ago, бm not sure if it was from в usb or в dvd but I did install it somehow)

So now гm basicly stuck with в персональный компьютер that boots automatically to memtest and I cant do anything.

Any suggestions?

ps - sorry for the bad English, it's not my native language.