Try Alt + Spacebar then type M and use the arrow keys to move the dialogue box back onto your screen.
In the future a tell tale sign for this is the symptom you describe and while the dialogue box is active but not visible, when you click on the screen with the mouse you get the default 'bong' sound generated by windows.
I suspect this is your problem.
For some reason this seems to be a common occurence with Inventor. Its happened to myself and my colleagues several times over the years.
Edited by: scottmoyse on Sep 16, 2009 8:53 PM
Scott Moyse
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Design & Manufacturing Technical Services Manager at Cadpro New Zealand
Co-founder of the Grumpy Sloth full aluminium billet mechanical keyboard project