I declined a single meeting occurrence in Outlook. How do I get it back?

I just had the same need today (restore a declined occurrence of a recurring meeting not organized by me).

The below procedure worked for me (Outlook 2013):

1. Go to the calendar view
2. Double click on a remaining occurrence
3. Choose to open the "entire series"
4. Edit the "Recurrence" of the meeting (Ribbon > Meeting Series > Options > Recurrence)
5. Do a fake change (move something back-n-forth in the settings with no impact on the schedule so that Outlook "thinks" you have changed something)
6. Close the Recurence dialog (click OK)
7. "Save & Close" the meeting series

Now all instances should be back.

I also could not "undecline," so I went to the sender's calendar, opened the item, and cllicked the button to copy to my calendar. A message popped up and said that to do this, I had to accept the meeting, which I did. Now it's on my calendar. Hope this helps.

It might be in your deleted folder.