I have an old Thinkpad T410, originally had Windows 7. Then I installed Linux Mint to dual boot. Later I made space and installed Ubuntu as my main operating system, but still have Mint and Windows 7 there. So far so good. Everything was as you'd expect, booting up would display the ubuntu bootloader with the ubuntu options top of the list.
Recently the bootloader changed although I have not done anything to change it. Now I get the following on boot
GNU Grub Version 2.02 *Debian GNU/Linux Advanced Options for Debian GNU/Linux ...
with the other boot options listed underneath and no mention of Ubuntu at all. letting it boot to the default "Debian GNU/Linux" option shows a Debian8 splash screen, then boots to ubuntu 18.
uname -a gives output as follows:
john@john-ThinkPad-T410:~$ uname -a Linux john-ThinkPad-T410 5.3.0-45-generic #37~18.04.1-Ubuntu SMP Fri Mar 27 15:58:10 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
I am completely at a loss to explain this. I've never installed Debian on any machine.