Autosum returns zero balance - why?
 

Excel 2003
I've been adding both contiguous and non-contiguous numbers using the
autosum function OR =c2+c5+c9 OR =SUM(c2,c5,c9) OR =SUM(a1:a50). In a few
places I now get a zero where once the total appeared. I've tried locating a
circular reference error but there isn't one. I've been using these formulas
for years and never had a problem like this before. What am I missing? Please
help!