I presume it was the cell containing =week that showed #VALUE
If you enter the formula =IF(DATA!$C5="","","WEEK-"&WEEKNUM(DATA!$C5)-1)
in another cell, at the same time as =week shows the #VALUE, does the cell containing the full formula also show #VALUE, or does it give a WEEK-xx message.
If it gives the #VALUE error, use formula auditing to see where the error is occurring.
Select the cell then:
In Excel 2003 goto 'Tools' - 'Formula Auditing' and select 'Evaluate Formula'
In Excel 2007/2010 from the Ribbon select 'Formulas' then 'Evaluate Formula' in the 'Formula Auditing' section.
Then in the Evaluate Formulas dialog box, click Evaluate.
Look at the part of the formula that is underlined, and this is what will be evaluated at the next click of the 'Evaluate' button.
Between these two items - comparing the result of the full formula and formula evaluation - it should narrow down the source of the problem.
Let us know what you get.