I have the following formula in an Excel spreadsheet:

=SUMIF('Placement List - Done'!$M$3:$M$1301,A13,'Placement List -
Done'!$L$3:$L$1301)

The A13 reference is a person's name; the data is basic sales information.

I have been using a series of linked spreadsheets as a make-shift reporting
database for 3 years. Up until today, it worked fine. This particular
formula does not go outside of the Workbook - just between sheets in the same
workbook.

Suddenly, it will not bring back any data! F9; re-entering formula;
re-formatting source data - none of this has helped.

Any suggestions?

Brian