Cannot find 3d curser - new objects frozen

I have found a strange problem and cannot find out what has happened.
I am building a scene, I have dozens of objects scattered over 4 layers as I slowly build stuff together.
I needed a circle plane, so Spacebar, Add Circle, click ok - but no circle. I get my zyx curser arrows in the centre of the starting grid, but no circle - nor can I move the curser. Even Grabbing and typing a value doesn’t work. The thing is locked and I don’t know what to do.
I have tried adding sphere, lamp, text, cube and I get the same thing.
I have lost my 3d curser as well and no amount of clicking does anything to bring it back. If I press C, my screen goes blank as I’m lost in a grey void somewhere.

Did you try Shift+C, this should center the cursor, if everything is allright.
Have a look in the ViewPropertiesPanel you can find in the View Menu of the 3DView.
There is the cursorlocation in numerical values.

And check out the outliner, because you have perhaps produced some objects somewhere in the grey outlands.