You can view deactivated participants on the participants widget. To begin, select Participants from the navigation panel on the lefthand side of the dashboard:

Once you are on the participant's list, first click on the More tab in the upper right hand corner. Then select Inactive. Finally, click on Deactivated. These buttons are shown below:

After you select these filters, you will see the list of deactivated participants. Importantly, next to each name is the reason for why they were deactivated.

To reactivate a participant, follow the instructions in this article.