SUM not working in Microsoft Excel

I am using Microsoft Excel. In that I found that SUM is not working at all. There is some problem with Auto Sum. Before I just choose the set of cells and then click on Auto Sum. The total appears at the end. But now the issue is bit weird. When I click on Auto Sum I am getting 0 as answer. It is not calculation it. While if I type the formula manually it works. Does anyone has idea on this.

Right click on that cell and then choose Format Cells. In that you have to go Number section and click on Number below General. Provide the decimal places if you want or you can leave that as 0. Once it is done then apply it and then try to check back. This kind of problem appear when the cell format is changed. You will need to restore that through Format cell.