Problems with booting the system

[Watermann]

First time this happens. At boot after a looooong time displaying a black screen, it suddenly displays the grub screen. I can select some possibilities but do nothing because I do not know what to select.
Then, again after a long time displaying a black screen, it suddenly boots and displays the login screen. Then after login everything seems to work normally.
I reinstalled a Timeshift backup from 4 days old version but that did not solve the problem.
Any suggestions what I can do to get things running normally again?

[Snowhog]

it suddenly displays the grub screen

What do you mean here? The Grub Menu, or the Grub prompt?

[Watermann]

What do you mean here? The Grub Menu, or the Grub prompt?

I get a grub display where I can select things. If I do nothing (I do nothing) then it starts automatically and after a long time with again a black screen, I get finally the login screen.