I couldn't perform a hard reset, probably because I did it wrong. But, by accident, I pressed next and prev page buttons at the same time, and it said sth like 'no new firmware found'. After that, I turned it off and pressed home button when switching on. I saw message in German language (quite weird, as evth was in English so far): 'Das Geraft laut im Sicherheitsmodus mit Standardeinstellungen. Um die Einstellunganderungen zu aktivieren, ist ein Neustart notwendig.' I guessed the meaning of this message, I turned it off and on again, and it works ok
The truth is, some books I copied recently are missing, but it doesn't matter at all.
It's probably my lack of knowledge
I think I should install the newest firmware, maybe it could save me some troubles.