How to restore dismissed reminders in Outlook 2010?

Your question asks to "retrieve all dismissed reminders". I am not sure if, by "retrieve", you mean to view a list of all dismissed reminders or if you want to "undismiss" them. I've been looking for a solution to the latter for a long time and just now stumbled upon it. You first open the meeting or appointment (I double-click the event in my calendar). At the top, you will see this:

If you dismissed the reminder then the Reminder field will say None. Change it to a new interval and then click Save & Close. You should get the reminder again.

It is not possible to restore already dismissed reminders.

But you can open the calendar view and search by modification date:

In Outlook 2007 or Outlook 2010 use modified:date format in the search field.

For example, I would type modified:2.10.2012 to see items I might have dismissed today

It's not 100% accurate as many things can modify the item, but if you do this soon after dismissing the reminders, it should be 100% accurate.