Updated Illustrator-Default Brushes Disappeared

There are no "default brushes".

Brushes come as part of a document profile you select.

So when you just open a PDF or an EPS or a raster image there might not be any brushes.

You only get brushes when you create a new document using the default new document profiles in Illustrator's "New document" dialog box.

In order to get brushes, load them from libraries. Access the libraries via the brushes panel (bottom left button)

Can you explain more details about your problem ?!
but if it is about resetting your app to defaults you can make that by:
Win: close your app then Press Ctrl + Shift + Alt immediately after you press to open your app again and hold pressing for seconds.

Mac: close your app then Press Cmd+ Shift + Option when you pressing to open your app again and hold pressing for seconds.

Hey

When you say "updated Illustrator default brushes" do youmean that you've added to them, but you're just seeing the regular default brushes?

If I'm understanding you correctly, that is normal behaviour. If you want to create a brush set that is there with every new document you create, you need to change the default document templates.

Hope that helps.