After syncing with Google account all my previous offline calendar entries are gone

I bought an Android 11 phone at the end of last year (the phone itself is a no name: "Kruger&Matz"). I started using pre-installed Calendar app -- strictly offline, with no syncing to any account, linking to any cloud etc. It worked perfectly for eight months. Couple of weeks ago (I may or may not have been a little drunk) I let my Google account to sync with all apps, including calendar. Since then, when I open the calendar, it's completely empty, all my previous entries are gone. (I checked my account's calendar's "Trash" folder on desktop, nothing there either.) The calendar stays empty after refreshing it, denying account's access, even when I completely remove all accounts from the phone. Is there a possibility to retrieve the older version of the calendar with all previous entries, or is it lost forever? (I haven't tried to disable or force-stop the calendar app.) Thanks in advance for all the answers and/or ideas. P.S. I really hope you understand my problem, since I'm quite illiterate when it comes to technologies, so I might have said something illogical or plain stupid. Thanks again!