You don;t have a Viewport pane loaded, that is why the background is grey.
Go to Window > Panes (Tabs) > Viewport, and dock the screen in the grey area. You must have closed it by accident.
If you can't do that, try changing the workspace layout to something else, then back again (Window > Workspace > Select Layout) that may bring it back.
Comments
You don;t have a Viewport pane loaded, that is why the background is grey.
Go to Window > Panes (Tabs) > Viewport, and dock the screen in the grey area. You must have closed it by accident.
If you can't do that, try changing the workspace layout to something else, then back again (Window > Workspace > Select Layout) that may bring it back.
Thanks Jimmy,that sorted it out:-)
Made my day