When you are invited to a meeting, Outlook does the following two things:
You can respond to the meeting requests by selecting one of the four options from either calendar or the invitation email:
It is highly recommended that you respond to a meeting invitation before you delete it from your mailbox.
If you have not purged Deleted Items folder in your mailbox, go to that folder and find the meeting invitation, then click "Accept" button in the message, the meeting will be placed back in your calendar again.
If you have purged Deleted Items folder within ten days or so, go to "Tools" in the menu bar, select "Recover Deleted Items..." option, from there first restore the deleted meeting invitation, then click "accept" in the message after it's recoved.
If meeting invitations are no longer recoverable, you have to set up the meeting manually in your calendar. Please note, since the meeting you set up is not linked to the original meeting, you won't be able to receive any updates or cancelation notice.