Ok, I've resolved it. Fortunately, everything is fine, that was
only issue with bootsector. Failure even before kernel load pointed me towards MBR, so I've started system with my pendrive kept for such occasions with portable win, linux and several tools, mounted my disks (first relief - filesystem was fine) simply
chrooted (please see edit below) to my regular root and called
and my system is alive again. I'm very curious what could made such strange error, I've already modified file mentioned in my question and of course it doesn't change anything, system starts with my changes and without them. I haven't touched anything connected with bootsector, or even anything in system's internals, so that's very disturbing, I'd really prefer if it was my failure.
Was that some error on my SSD? Or some cosmic rays? Or maybe virus? I don't know, if anyone has any idea how to reproduce that error I'll happily try it.
Edit: As @peterph wrote, "it is a Good Idea (TM) to bind-mount
/proc into the appropriate places in your root partition before chrooting into it" - thanks for noticing that.