If you are missing events from one, more or all calendars in your calendar views, this can be caused by (accidentally) using the wrong profile. Profiles filter your calendar views to only a subset of your calendars, see Introduction to profiles for more info about profiles.
You can directly see if you are not in the default ALL profile: then the menu icon in the top left corner is replaced by a circular profile icon.
ALL profile (menu icon shown) - all events from all calendars are visible
ACCOUNT profile (circular account icon shown) - only events from the selected account are visible
CUSTOM profile (circular custom icon - emoji or first letter - shown) - only events from the configured calendars are visible
To manage profiles, open the main menu (tap on the menu icon or profile icon) and then press the account header on top and select manage profiles.
If you accidentally changed the profile and you are not using profiles, please switch back to the ALL profile and make sure the ALL profile is configured as default profile (marked bold). You may also have configured a widget in a different profile and have set the widget to open the app in the widget's profile.
If you are using profiles, please adjust the profile to include all required calendars.