Docking the window in InDesign

(1) If you have set up a workspace, go to Window > Workspace [at the top of menu] > Reset [Workspace name] should return things to normal.

(2) If that doesn't work, restoring your preferences and caches should also return the interface to normal. Here's how to do this:

https://www.rockymountaintraining.com/adobe-indesign-rebuilding-preferences-cache/

If the standard ways described on the web are not working (that is, you don’t see the blue line when dragging the document title bar near the top of the workspace), then the first thing to check is to look at the bottom of the Window menu, and see if Application Frame is enabled. Docking a document window works only if the Application Frame is used. If Application Frame is disabled, all document windows float and are not dockable.