Copy cell S79 into S80, S81 etc and see if it will now work.
(wouldn't bother going all the way for now. I've had formulas stop working like this on odd occasions for no apparent reason and re-copying has solved the problem.
Must say that sometimes the problem has been caused by faulty date or formatting of a cell somewhere else in the spreadsheet and if you are working in the S column this one sounds quite large!
Just tried typing this formula in and copying it a little way. Worked okay until i entered a 0 value in the A column and then it failed from the next row onwards. Have a look at what is entered in the a79 cell - delete if necessary and re-enter. Check the format of the cell - is it still a number?