I'm getting this error message when trying to log in to Workbench:
OAUTH_APP_ACCESS_DENIED: user is not admin approved to access this app
This is a recent issue as I have been able to access Workbench before
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Workbench uses OAuth, which creates a connected app in your org. Your Salesforce Admin has the ability to limit access of certain connected apps, or even turn off access altogether.
I suspect what you'll find is that someone has done precisely this, and disabled access for workbench to your org, or perhaps for your profile in your org.
If you're the Admin, you can see how to manage Connected App access here in this help topic.
Here's a screen shot of a DE org I have with lots of connected apps, including workbench to see where you'd see this:
If you click on the connected app for Workbench, you can then configure the access settings.
You'll see the OAuth policies setting. If you edit it, you can adjust it. If set to All users may self-authorize, then anyone can use that connected app.
If set to Admin Approved users are pre-authorized, then you must have a pre-authorized profile or permission set assigned to you as a user in order to use the Workbench connected app.