In SQL NULLs propagate - so if you add a column that contains a NULL, you will get NULL as the total result:
SELECT 1 + NULL + 2
So if you are performing any aggregate operation on your column, and a NULL value exists, you will get a NULL result.
One way round this is to use ISNULL in your query:
SELECT SUM(ISNULL(MyColumn, 0)) FROM MyTable
Will treat all NULL values in the column as zero.
If this isn't the problem you are having, then you need to explain in much better detail exactly what you problem is.