So you get something like =SUM(A1:A10) but this displays 0?
Sounds like they are not really numbers but text
Try
1) format the range as numbers
2) copy an empty cell, select the range of interest and use Edit | Paste
Special with Add specified
--this trick converts text digits to numbers most of the time
3) if all else fails, after formatting the range as numbers, retype the
numbers
best wishes