iOS-18 made improvements to calendar and it was after 18.5 that I had my moment of calendar "fear" when noticing events over 3 months old (or was it 6?) were no longer displayed, which had me believing all my events were gone, forever. Are you saying all your events, even yesterday's are gone? Have you gone to Settings, Apps, Calendar, Sync, and made sure "All Events" is set?
Actions like "Restore iPhone" and "restore backup" have a place in fixing problems when applied correctly for the right situation, but restoring the same setting causing the situation as I experienced was not going to work, the setting had to be changed.