If you've been invited to an Outlook event and accidentally deleted it from your calendar, you can follow the steps below to restore your deleted event.
Please note that this method applies only to when you've deleted an event you've been invited to as shown in the image below, and not to events you've cancelled where you're the organizer of said event.
Head over to your Mail section in Outlook
Navigate to the Deleted Items folder and locate the event you'd like to restore
From there you can RSVP with "yes" which would add the event back to your calendar