Undersize window display on monitor

For unknown reasons, the worksheet display on the monitor
consists of two windows. The inner window is reduced in
size(both horizontal and vertical) to about 85 percent of
a full window. The normal size display also exists with
all menu items etc in the normal size, but within what
would normally be the spreadsheet display area is a second
window, complete with title bar (but no function
menu). There is no display in the right upper corner
of this inner window (ie, dash, box or X) to permit
returning it to normal size. All normal functions are OK
such as printing or modifying the worksheet. This workbook
includes 53 worksheets, each with extensive data, so it
would be a problem to copy these to a new workbook with
the normal display characteristics. Any suggerstions
would be very much appreciated.