As I was working on my document I must have struck a random key, which caused inDesign to create a new workspace smaller than the one I had opened. No idea how that happened or how I can close the new instance. What I will try next is to close inDesign and restart, but would nonetheless appreciate a hint about what may have caused this.
Try Workspace > Reset [name of your workspace]
thanks, will try this when and if it recurs.