How can I retrieve the shared calendar I deleted accidentally?
If you have deleted (unsubscribed) the calendar that was shared to you, you can get back the deleted calendar.
1. If the calendar is a public shared calendar, request the calendar owner to email you the Calendar URL. Using the URL received by email, you can retrieve the shared calendar. This is achieved with the "Mail to" option provided in the calendar settings page.
2. If the calendar was privately shared with you earlier, request the calendar owner to email you the calendar URL using "Sharing and permissions" options under calendar settings page.
Refer here to learn more about the ways of sharing your calendar.