Excel 2003 s/s AutoSum not responding

I have been entering data into Excel 2003 s/s with column B containing currency amounts using the AutoSum formula to get running totals as each row is entered. This has been adding the totals OK until the present cell B67. Suddenly the AutoSum has stopped adding additional entries. If I click on the cell showing the AutoSum Formula it shows "=SUM(B8:B67)" i.e. without quotation marks. If I delete row 67 the total decreases OK by the deleted amount and the SUM Formula adjusts accordingly but nothing changes if I attempt to add an amount to Cell B67 , i.e. it remains at the deleted total. This has just happened today when I resumed adding data to this s/s. The only change I did yesterday was to add a fourth column which contained Dates of new data entered which I was not in possession of for the previous entered data. Can you explain this failure of the AutoSum Function here ?

Thanks to all who replied here. I found that the SUM FORMULA had not been installed far enough down in order to include all the ROWS required. Apologies for bothering everybody with such an obvious mistake !