Adding up a column keeps coming up with 'value!'

I’ve been working with this spreadsheet for months and it now refuses to add up my column of numbers and keeps showing ‘value!’. I’ve tried putting in the formula again, reformatting the column as numbers and a few other ideas found on forums bit to no avail… any ideas?

Do not horizontally align your numeric columns explicitly. Then you will clearly see if there are numbers contained. (Texts left aligned, numbers right aligned.)
Formatting of a column “as numbers” does not convert text to numbers!
We would need to see a piece of your spreadsheet showing the issue.
Please provide access to a respective .ods file.

For quick test to check if reffered cells contain numbers or text strings, you can use View -> Value Highlighting from menu. Numbers font will become blue, formulas - green, text - black. In any case, as @Lupp mentioned, example file is needed to provide correct suggestions, not guessing.