SUMPRODUCT help

I need help using the SUMPRODUCT function.

Firstly, I need to sum the total number of blank cells between a certain defined week range. For example, Sheet1 will display totals, Sheet2 has the data to be counted. I need to count blank cells in Column O (Sheet2) from a date in Column A (Sheet1), to a date in Column B(Sheet1). Secondly, I need the answer to this displayed on Sheet1. Using SUMPRODUCT I've only been able to return #NAME? or #VALUE?

I'm completely stuck and have been trying for the better part of three hours. Any help is appreciated.

Perhaps something like:

=SUMPRODUCT((Sheet1!$A$2:$A$100<=A2)*(Sheet1!$B$2:$B$100>=A2)*(Sheet1!$O$2:$O$100=""))

Where A2 on active sheet has date to check for... adjust ranges to suit, but don't use whole columns.