On the Top Menu go to Poser > General Preferences ...

ON the Interface tab put the dot on Launch to factory state.

[OK]

Exit and restart poser.

That gets you back to the factory UI. You can safely return the preference to "Launch to previous state" in preferences.

Now how to make sure this never happens again ...

Every room in poser has a set of memory dots. Among other things they store the UI layout for that room.

A Dark dot is empty.

A light dot contains a stored layout

A blue dot is the layout that's applied.

Click a dark dot to store a layout

Click a light dot to load a layout.

[ALT] + Click a dot to erase what's stored in it.

Once you've reset your UI you should store the factory UI in one of the dots.If it ever gets messed up a gain your one click from the factory layout. Then arrange your pallets to your liking and store that in another Memory Dot. Next thing you know you'll be storing different UI arrangements for different tasks.

Every thing about Memory Dots: https://www.posersoftware.com/documentation/12/index.htm#t=Poser_Reference_Manual%2FBuildingScenes%2FPoseRoom%2FMemory_Dots.htm&rhsearch=memory%20dots&rhhlterm=memory%20dots%20dot