With the new permission system, it is possible to remove project administration permission to any user, including the global administrators. (As a reminder, on 5.6, the global admins always retain project admin permission on all projects).
As a consequence, if that is done accidentally, someone (a global admin) would have to restore project admin permission to at least one user. Unfortunately this is not possible to selectively restore project admin permission on top of existing permission.
The only way possible to do that is to re-apply a permission template that grants project admin permission to someone, overriding all existing project permissions.
Add a new menu item in the global project management page to be able to restore access permissions to a project (browse + admin)