Links Must Be Open?

I have one spreadsheet pulling data from another but it says the links must be open to get the data. This isn't needed on another spreadsheet which does the same thing. Any ideas what the problem is? It might be that it's a large file. Thanks.

What functions are used in the formulas that reference the external links? Some functions (e.g., INDIRECT) don't work with closed workbooks.

In the first report I just use the "=" function. It works normally but not today. Thanks