On my laptop running Ubuntu 14.04, I usually attach a secondary monitor to it while working. I notice that on disconnecting the secondary monitor, some windows tend to go offscreen making it impossible to use them. Pressing the usual Alt + Tab plus clicking the window doesn’t bring it back. So here is a hack for bring back all offscreen windows which I found on Stack Overflow.
Go to Appearance -> Behaviour, then check or uncheck Enable workspaces.
Reference
linux – Window appears off screen on ubuntu – Stack Overflow. http://stackoverflow.com/questions/12477435/window-appears-off-screen-on-ubuntu. 19/12/2016.