When I open up my workbook I get an error that data has been lost. I have
isolated this to a worksheet where I have this formula.

=NETWORKDAYS(L5,M5,$K$20:$K$24)

The cell has the # of net workdays listed but when I click on the cell and
check the formula bar, "#NA" is in the formula bar and not the formula I
listed above.

I correct this and save the book and it seems to be OK for awhile and then
suddenly I get the lost data error again when opening the work book. Any
suggestions?