The address bar is probably still there, timke, it has just been moved. Right-click on the top toolbars and untick the "Lock the Toolbars" option (it will probably be unticked at the moment anyway). Then look around the toolbars for the word "Address", (it will have a couple of little chevron arrows beside it). If you see it, click and drag it down and your address bar should re-appear. Now right-click on the toolbars again and tick the option to lock the toolbars, that will stop it happening again.
Pressing F11 simply expands the window to fill the screen area. As you suggested Maj,the lock toolbars option was unchecked.I am unable to find any sign of the word "address",with or without chevrons. Luckily I am in no desperate need of a result as I use Opera as my default browser. Thanks for the suggestions.
Just remembered that if you Unlock the Toolbar, you can drag various parts of it around and up and down - this once "rediscovered" the Address Bar for me on a friend's system. She had also "lost" the address bar even though it was checked.