I have some references in a Main worksheet that link to another spreadsheet
just to populate it with data, no complicated formulas.

=LOOKUP(E2,'[NHP Animal Census Data File.xls]Clinical'!$A:$A,'[NHP Animal
Census Data File.xls]Clinical'!$C:$C)

Now when I open the Main worksheet the I get #REF! all the way down the
columns. Only if I open the other spreadsheet does the information populate.
I never had to open the other spreadsheet before now. It had been working
fine for months. Nothing has been changed as far as the formulas are
concerned. In fact I had a backup copy from a few weeks ago and the formulas
are exactly the same. Any ideas of what could have caused this and how I can
fix it?