This message can appear for the following reasons:
Microsoft Excel is closing and there is an invalid value in a cell
An OLE operation has not completed, such as an ActiveX control or an embedded program or object
Excel is in print preview mode
A Microsoft Visual Basic macro is running in step mode (a form of testing and debugging that takes place within the VBA editor)
A file is attempting to load in Excel
A dialog box is open, usually behind the Excel window
To avoid this message, allow time for pending tasks to complete, close any open dialog boxes, close the Visual Basic Editor, and close any other programs that use objects embedded in the workbook.
If the message still appears, press CTRL+ALT+DELETE, and then click Task Manager. In Windows Task Manager, click the Applications tab, select the open workbook, and then click End Task.
REFERENCE IS NOT VALID