You just did hit the Tab key unintentionally 
hit the Tab key again (above [Caps Lock]),
The [Tab <->] key is a toggle to hide / show all dockable dialogue tabs/windows (very useful when you need more space for your canvas, like while working on a highly zoomed image)
Image Wikipedia https://en.wikipedia.org/wiki/Tab_key
hit the Tab key again (above [Caps Lock]),
The [Tab <->] key is a toggle to hide / show all dockable dialogue tabs/windows (very useful when you need more space for your canvas, like while working on a highly zoomed image)
Image Wikipedia https://en.wikipedia.org/wiki/Tab_key