If you want to retrieve one reminder for a specified calendar items, you can add reminder for this calendar items again in Outlook. Please do as follows:

In the Calendar view, (1) click to select the specified calendar item whose reminder you will undo dismiss to activate the Calendar Tools, and then (2) choose a new reminder from the Reminder drop down list in the Options group. See screenshot:

One click to remove (or restore) all birthday reminders in Outlook

With Kutools for Outlook's Remove Birthday Reminders feature, you can easily remove all birthday reminders from your Microsoft Outlook with only one click. Kutools for Outlook also provides the Restore Birthday Reminders feature to quickly restore all deleted birthday reminders with one click.

If you want to retrieve multiple dismissed reminders in an Outlook calendar, you need to add the Reminder column into the calendar folder, and then quickly add reminder for each calendar item with one click.

1. In the Calendar view, select the specified calendar folder where you will retrieve multiple dismissed reminders, and click View > Change View > List. See screenshot:

2. Go ahead to click View > Add Columns. See screenshot:

3. In the Show Columns dialog box, please do as below screenshot shown:
(1) Select Reminder in the Available columns list box;
(2) Click the Add button;
(3) Keep the Reminder column selected in the Show these columns in this order list box, and click Move Up or Move Down button to change its order;
(4) Click the OK button.

Now, return to the calendar folder, and you will see the Reminder column is added in the folder.

4. Type the search criteria modified: today into the Instant Search box to quickly find out all calendar items which are modified today.

5. And now click each cell in Reminder column to add reminder for every found calendar item.

Note: This method will add default reminders to all calendar items modified today.

Undo send (unsend) a sent email in Outlook

Undo delete (recover) deleted emails in Outlook

Clear offline items and recover (undo) in Outlook