That .ics file won't open (or lands in the wrong place)
An .ics file is plain text describing events. When double-clicking does nothing useful, either no app owns the .ics extension, the app that owns it isn't the calendar you use, or the file itself is malformed. Importing from inside your calendar app, rather than double-clicking, sidesteps the first two entirely.
Work through these, most likely first
1The wrong app owns .ics
On Windows, .ics often opens legacy Outlook; on Mac, Apple Calendar claims it even if you live in Google Calendar. Don't fight the default: in Google Calendar use Settings > Import & export; in Outlook use Add calendar > Upload from file.
2The file is malformed
Files missing VERSION:2.0, missing END:VCALENDAR, or using bare newlines instead of CRLF line endings get rejected by stricter apps, often with no error message. Run the file through an ICS validator to see exactly what's wrong.
3It imported, but into a calendar you can't see
Imports land in whatever calendar was selected in the import dialog. Check the calendar list sidebar and make sure that calendar is visible.
4You wanted a subscription, not an import
Importing a file copies events once, frozen at that moment. If the schedule changes later, your copy won't. For anything that updates (team schedules, school calendars), subscribe to a feed URL instead of importing a file.
Rarer, but worth knowing
Timezone surprises after import. Events defined in floating time or with nonstandard TZIDs can land an hour off. That's a source-file problem; the fix belongs with whoever generated it.
Huge files. Some apps time out importing thousands of events at once. Split the file or trim old events.
If someone keeps sending you .ics files for a schedule that changes, move the schedule to TangoHive once: import the file, fix it if needed, and share a live link and feed instead of re-sending files.