Maybe the window is just off the visible screen.
In xl2003 menus, I'd use:
Window|Arrange|Tiled
and resize it the way I like.
Then save the file--so it's nice the next time I open that workbook.
If you're using xl2007 and opening the file by doubleclicking on it in windows
explorer...
You are suffering from a common problem in xl2007 that lots of people have
complained about. The workbook is open, just not visible.
One way to see it is to click on the Office button and start to open another
workbook, but then just dismiss the dialog. You may see your workbook open
after this. (Ctrl-O, then escape may work???)
Currently, there isn't a fix--just workarounds.
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I've seen some posts that suggest that turning on the solver addin (or any
addin) may remove the problem. I've never experienced the problem, so I never
tested if this was a solution.