A host / super user account. Hosts have full permissions to all sites in the DNN instance.
Steps
Go to Host > Advanced Settings (double-gear tab) > SuperUser Accounts.
Search for the deleted user account.
You can use the Search textbox and dropdown at the top to search by a user account field, such as Username, Email, Display Name, Street, City, social media account, or Preferred Locale.
You can also filter the displayed accounts by clicking/tapping Deleted.
Click/Tap the back-arrow icon for the deleted user account to restore the account.