I'd guess you had a typo.

You may want to post the way you did the hyperlink (insert|link or
=hyperlink()???).

And what your hyperlink address was.

(Don't have any octothorpes (#) in the filename/sheetnames.)

Frustrated Engineer wrote:

I created a hyperlink in A Excel file to B Excel file. It referenced a
bookmark in B. It worked when I tried it before I closed it and it worked.
When I opened A again and clicked the hyperlink, it said "The address of the
site is not valid...Check the address and try again."

--

Dave Peterson