There are many reasons and ways to have the links lost. Equally many to fix. I don't know your workbook, so I can only suggest few things to look at/try before you go crazy
1. Try to find what it links to. If the workbook(s) came by mail and you placed one somewhere, the linked workbook may still be where your mail goes, for instance c:\temp. it would be best to find it, open both books, break the link if you think you don't need it by doing copy and paste value in the cell(s) using the linked data.
2. Similar scenario might be that you copied 1 of 2 workbooks someplace, and the original is still sitting there. Are these your files?
3. On the Insert menu, review named areas. Some may tell you exactly what workbook is linked. The linked workbook name will be in square brackets [ ].
4. On the Edit menu, review the link itself - it tells the path just before it got lost.
5. If you can't find the linked file, the hard way is to set the worksheet to show formulas only instead of normal values, and read the formulas which might well contain the information. The setting is on the options menu.
Do not delete anything, because the spreadsheet is unlikely to work if you break the link.