Excel 2007 : Excel 2007 Formula stopped working

I have been using the same Excel 2007 workbook since January containing a number of formulae, ranging from simple (=D3) to multipage Counts and If's (=IF(G39=" "," ",LOOKUP(G39,'Completing 13jun'!$X$33:$Z$57)). Now, although the formulae still show, they don't compute. I have tried going back into old sheets and enter new data but the formula doesn't work. Any ideas?
Van