all events disappeared from calendar after High Sierra upgrade

Found the (a) solution myself, after trying to import the .ics files one by one to no avail (they were all holydays and birthdays). Anyway: you should locate the "Calendars" folder that can be found here: Macintosh HD > Users > "your-username" > Library; bear in mind that it is invisible by default, and to be able to visualize it you should have your User folder opened/selected and go to the Finder menu > View > View Options > tick the last box at the bottom of the list beside "Show Library folder". Then, I copied the "Calendars" folder on the desktop (don't know if this is necessary, but since I was just trying things I didn't wanna corrupt the original files or mess with their original location): inside the copied folder you should search for a file named "Calendars and Reminders <date and time>.icbu". Once you found it, you can launch Calendar: in the top menu you select File > Import > and, in the browsing window, the .icbu file inside the Calendars folder that you copied on the desktop. A little progression window with the blue bar will appear to show the progress and, once it's finished, all your events will be back in their right place. Hope this will help someone 😉