I have complete confidence that this problem will be resolved by restarting, however, it seems very odd that it should disappear randomly during my session.

To put it simply, I've lost the top tray-bar and the side panel disappears when the mouse moves within touch, however if it click and drag the window by the side it comes back (but disappears instantly when touched again (however I can still click on the buttons where they should be).

The only guess I have as to why it's happened is a bug, or I've lost a file these GUI things depend on.

This is a real pain as some applications now have lost the "File, Edit" menu items as a result of this.