Just to add to Harlan's response...
Styles live in workbooks.
If you haven't made a change to the Percent style in a default workbook (like
book.xlt stored in your XLStart folder), you can try this:
Start a new workbook (not an existing workbook)
Open the workbook with the problem
Make sure the problem workbook is active
Format|style|click the merge button
Point at that new workbook (book1, maybe???)
Then finish up that dialog, close the new workbook and test out the Percent
button.