I assume your talking about within Excel. Ctrl+F10 will restore. So if you
minimized the workbook, Ctrl+F10 will restore it. Other than that, there is
no shortcut within Excel that Maximizes.
You can use the following code to create a macro that would do this for you.
Then assign the macro to a shortcut.
Sub Maximize()
'
' Macro recorded 1/26/2006 by Cooper
ActiveWindow.WindowState = xlMaximized
End Sub