The workbook appears to be locked. You cannot enter data in any cell including those presented in blue or purple.
This applies to Excel 2007 only.
Your workbook may be “Marked As Final”
To check the status of this feature, click on the Office button in the upper left corner. Choose “Prepare” then look for “Mark As Final.” This feature toggles on and off. If on, then the icon is presented with an orange background.