This may be because you are trying to edit and save a document that is in "read only" mode - this could be because when you saved the last edit in save as you opted for the read only mode when re-opening.
You may be able to change this back by..
Make sure that the file is not open in Excel. On the Windows Start menu, point to Programs, point to Accessories, and then click Windows Explorer. In the Exploring window, open the drive or folder that contains the file. Click the file name. On the File menu, click Properties. Click the General tab. Clear the Read-only check box
Another alternative is to save the edited document under another name making sure that you do not choose the read-only option.