the admin role is removed for system administrator

[SaidaraoD]

If the admin role that was added by default when the PDI was created is removed, is it possible to add it back to the system administrator?

[Zach Koch]

You would need to find the other users who have the admin role and give it back to the admin. If you look at this KB article KB0857988

It shows that the system has a property OOTB that enforces that 2 users, at a minimum, have to have the admin role in the instance. So the only reason that it was allowed to remove admin from the Sys Admin user is because 2 other users must have had the admin role. I would go to the sys_user_has_role table to find out who has the admin role.

[SaidaraoD]

As per your requirement, the system administrator would run into trouble if they couldn't locate the user with the admin role while using this table, sys_user_has_role.

The specified page cannot be accessed due to security restrictions

[SaidaraoD]

I tried to utilize the sys_user_has_role table as you advised in order to determine who had the admin role, but I ran into trouble. Access to the requested page is restricted by security measures.

[Zach Koch]

So since you don't have admin you won't be able to view it. Try the steps in this thread

I lost admin rights of System Administrator in PDI 

If that doesn't work, then try opening a ticket with ServiceNow.

If that doesn't work you may need to reset your PDI