AutoSum

Well, I know of a couple reasons for that behavior.

1. Is the cell where you're attempting to enter
the AutoSum immediately adjacent the the cell range
that you want summed? For example, if cells A1:C1
contain {1,2,3} are you trying to enter the Auto-
Sum in to a cell other than D1?

2. Are all the values in the cell range that you're
trying to sum really text values -- not numeric values.
For example, does cells A1:C1 contain {"1","2","3"}?
The best way to check this is select an empty cell,
type an equal sign (=), select the cells to be
summed (e.g., A1:C1), and press Ctrl+=.