I'm not sure if I triggered something inadvertently but when I attempt to open some dialogs from the main screen, like help, or clicking on the style and selecting 'Choose style at any bar', that new dialog opens up behind the main screen so I can't see it unless I minimize the main screen.
Any idea what I might have done?
Thanks.

You likely have the original DAW mode enabled.

I concur with Jim's suggestion. Press the button marked 'DAW' to see if that resolves. There have been many requests for a better indication when DAW mode is enabled.

Thank you!

For the future, what you did is activate DAW Mode, then you maximized the BIAB window.

The DAW Mode is designed for a specific purpose, to give you a small window that stays on top so you can drag tracks to a DAW running in the full screen space underneath.

As a workaround, you can use a set of skins I created that will highlight the DAW mode when it is operating.

This post has instructions and screen captures that show the effect it provides.

Hope this helps!