How to restore (put back) a Calendar event from Trash in Outlook
For example, if you deleted a meeting from your Calendar, you can't simply Undo this action. To restore the removed event, do the following:
1. Find the deleted meeting in the Trash folder, for example:

2. Do one of the following:
- Open this deleted meeting and in the Meeting dialog box, on the Meeting tab,
in the Respond group, and then:
- If you have been accepted these meeting, select the Accept drop-down list and
click Do not Send a Response:
- If you didn't answer for this meeting yet, answer like you want.
- If you have been accepted these meeting, select the Accept drop-down list and
click Do not Send a Response:
- Right-click on this meeting and select Move -> Calendar in the popup menu:
- Right-click on this meeting and select Move -> Other Folder... and then in the
Move Items dialog box, select Calendar:
See also this tip in French: Comment restaurer (retourner) un événement de calendrier à partir de la corbeille dans Outlook.