I have pressed something to make one of my windows disappear. Could someone help me get them back?
Shift+Spacebar or Ctrl+UpArrow or DownArrow will toggle from multiple windows to fullscreen (only one window.) They are all toggles, so pressing any of them will get you back to multiple window view, if that was the problem.
However, the Information Window at the top of your screenshot is also a window, and should not be visible in fullscreen mode. Perhaps you merged one window with another? If you right click on the boundary between windows that can be merged (similar size edge) then select “Join” two windows will merge into one. You can get the window back by right clicking on the boundary or edge and selecting “Split” and remaking the window.
Slide your Mouse Pointer from the View and Controls buttons down to the grid. At the border, where the pointer turns to a double-arrow, rightclick and choose “Add Header”. Then use the same double-arrow on the same border to slide that window up with Left-Drag.
If that’s not what you lost you can use Ctrl-Up/Down arrows to toggle between fulscreen windows and splitscreen, the window with the mouse pointer in it will go fullscreen. Ctrl-Left/right arrows cycles thru Screens.