Vlookup stops working???

I use this formula frequently and have had some trouble with it. Recently, I
opened a file that uses this formula and got a message that said "Unable to
read file". The formulas did not work this day alsthough I have been using
these files successfully for a couple of years. My first attempt to correct
it was to open the linked file - this usually refreshes the links - but no
success there. Next, I checked the status of the links using "Edit/Links" -
the status was "OK" even after the linked file was closed - but no luck with
the formulas. Next, I "changed the source" of the link and pointed it back
to the linked file - no luck. My solution was to create new files using the
"Index/Match" combination in my formulas to get the same results. This
worked fine. Today, I reopened the files with "Vlookup" in them and they
work fine. [Excel 2003 SP3] If anyone knows the root cause of this....I
would love to hear from you.