I had a password.
I needed to change that password.
I went as root, wrote "passwd myname". And wrote my password.
I received something about "Segmentation Error".
I decided to ignore it (on the other hand it was probably very stupid).
It kicked me out "Password not altered".
And I attempted to reenter password (again "passwd myname" etc.). Which worked.
I "quit" root access in terminal, then I "sudo su" and I verified that I've written this password correctly.
And yes, the password got updated, then I went on with my life.
And then I had to move. I turned off my laptop.
At home, I opened my laptop. Input the password, and Linux crashed (partially).

I use Plasma KDE. I get regular account screen. I go to my account, fill in my latest password. AND, it crashes. The time doesn't go onward (I checked with phone, literally 5 minutes delay not a single change), can't select anything, only mouse is moving (but doesn't react on hover, clicks or anything, and its a cross (which means "wait" or "loading")).

I have some important stuff on this Linux, and I've encrypted my drive through options during installation. So I can't really draganddrop.

I can't login, I don't know what happened. My password consists of capital letters, minor letters, "/" and numbers. It would be cought by regex [a-zA-Z0-9/] It's length is about 15 characters.

Help. How did this happen? How can I fix it?

There seem to be solutions but they seem to far from mine.
- I didn't update Nvidia drivers. And even then I have integrated chip. If one would fail, wouldn't the other pick it up?
- I don't think I've updated kernel lately (that would affect this error). - I didn't install packages at that time. And the time I did, already did couple full restarts, so why wouldn't it crash back then, but now?

I always use latest version of Ubuntu (it tells me to update, so I update) and switching versions in grub doesn't change anything.

====== SUGGESTIONS FROM CHAT ======

Suggestion #1: Can you login using Ctrl+Alt+F1?
Yes. I pressed that, used my username and my password. And I managed to login through this console. No crashes. "LS" revealed "desktop-data.desktop" (or something like that) and another file. Which makes sense since I encrypted the drive. But I did manage to login.

Suggestion #2: Can you check memory?
I can't. I can't see option for this. Not even in recovery menu.