If you cannot see participants, there are several ways to try and do so.

1. Permission Settings: Perhaps you or the user trying to access the participants does not have the permissions to see them. If you are not the IA administrator, please contact them. If you are the administrator, contact support to get a list of users and their respective permissions.
2. See if participants are inactive: Follow the instructions in this article to view deactivated participants. There will be a reason given next to each participant name for why they were deactivated. If they are in-active and you would like to reactivate them, follow the instructions in this article.
3. Filters Applied on the Participant List - Check to ensure the correct filters (if any) are applied to the participant list you are viewing. See this article for information on using the Participants widget