03-25-2014 09:54 AM - edited 03-25-2014 09:55 AM
The Excel document may be improperly formatted. To get an idea of how the Excel document should be formatted, do a lookup of some of your existing contacts (or if it's a new database, use the demo), then click Tools> Export to Excel. This will open a document that is formatted the way it should be, and you can use that to compare.
Make sure you don't have any columns or rows that are absolutely blank.
For more information on importing from excel, see the following knowledgebase article: