Hey 
Ashley you know when there's a popup message on C3, like those when you change the Zoom in the browser and C3 gives you a pop-up saying "you'll need to restart C3 to restore the aspect ratio....." (or something like that) then in those cases the background is greyed out and you can only click the "ok" buttom on the popup, the editor is only active again after closing the pop-up window, so in this issue we are getting the same type of grey screen, but there's no pop up or anything to close.
With me it happens when I go back to editing after using the preview layout mode on another tab. The editor is not active, cant touch anything, not even choose a event sheet, object, anything. See the images attached here
see how the colors are dull like the editor is not active? its frozen...
its still happening to me and really annoying...
heres the pop up window example:
It seems to be a bug where the editor cannot recognize it should be active again
normally, when you go back to the editor tab after using the preview layout mode, there's a fraction of second where the editor is still "grey" and then turns active again, so you can keep editing, but with the bug there's no change, it remains inactive.....