Hello @Damus
In this case, every reduces to how the administrator rights are configured to the account and if there are other policies applied (such as Local Policies) or even some security softwares that may be installed on the computer. It would help to know how you did perform that change into Admin user, for example.
To discard other applications limiting your access in the computer, please try a clean boot: https://support.microsoft.com/en-us/topic/how-to-perform-a-clean-boot-in-windows-da2f9573-6eec-00ad-2f8a-a97a1807f3dd
Then, First of all you need to ensure that your account is member of Administrators group: Go in "Edit Local Users and Computers" > "User"s, then doubleclick the username of your actual account and in the "Member Of" tab should be only Administrators. Any other group membership would limit the access of this user.
Also make sure that the Administrator account rights are Enabled:
-Click Start and type command in the Taskbar search field.
-Click Run as Administrator, type net user administrator /active:yes, and press enter. Wait for confirmation and restart.
If you don't have access to this options, and you don't have any other username that has Administrator rights to perform this actions, I am afraid the only option would be format the computer. As recommendation,always keep a Admin user with the option "Password never expires" in case of emergency, at the same time you can use an account for everyday usage with or without admin rights.
Hope this helps with your query,
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