After you upgrade to the new Input System, the Standalone Input Module won’t work anymore. So, after upgrading, if your UI suddenly stops working, and you’re not sure why, chances are you need to replace your old Standalone Input Module with the new Input System UI Module.
To do that, find the old Standalone Input Module in your Scene, you’ll likely find this on the Event System game object that was added with your Canvas.
You’ll see an error on the component, with a prompt to replace it with the new UI Module.
Click the button to replace the component and your UI should work as it did before.
If your UI suddenly stops working after switching to the new Input System, This is why.
For more information, see the full article on how to use the Player Input Component.