Someone else may correct me, amaka, but I do think it's obvious. Your SUMIF function is looking at the values between rows 12 and 23875; but after you sort the data, I suppose that there are a different set of values in that range. Perhaps one of the values that was in row 33 is now in row 8, so it's outside the summed range; and some value that was in row 35133 is now in row 8305, so it's now inside that range.

I can't see your data so I can't be sure. But that's the first thing I think of.