Library view fail to return to develop module

[DK Sebastian]

Hi does anyone know that why I got stuck here once I strat LrC and couldn't return to the main menu page?

I couldn't select any photos or enter develop module either.

[Michael J. Hoffman]

You've hidden everything. Try pressing "Tab" or else click the triangles at the left, right, top and bottom margins of the window.

[Michael J. Hoffman]

Actually, "Shift-Tab" might do better - the shortcut to Hide/Show all panels. Tab just hides/shows the side panels, but you have the top and bottom hidden as well.

Another one to keep in mind is F5, which shows/hides the module picker. 

https://helpx.adobe.com/lightroom-classic/help/keyboard-shortcuts.html