My Excel workbook keeps crashing and I need help!

I have an extensive work book that containers many pages and links that allow us to navigate between them all. It keeps crashing lately and we are unsure as to why. The error starts when trying to open the file and gives the error "Excel found unreadable content in 'usabook21.xls'. Do you want to recover the contents of this workbook? If you trust the source of this workbook click yes" . Ic hoos yes then get "Repairs to 'usabook21'- Errors were detected in 'usabook21' but Microsoft excel was able to open the file by making the repairs listed below. Save the file to make these repairs permanent." Then below that reads "Damage to the file was so extensive that repairs were not possible. Excel attempted to recover you formulas and values, but some data may have been lost or corrupted" Then it gives me a link to click to view the log file listing repairs, that reads. "Microsoft office Excel File repair log. Errors were detected in file (file destination) the following is a list of repairs: Damage to the file was so extensive that repairs were not possible. Excel attempted to recover you formulas and values, but some data may have been lost or corrupted." And then after all that info if I click the close box on the error box it brings up all the data of the workbook but no formatting or links. Any idea what might be going on or how i may fix it? We copy the file regularly and have been loading a working version until it goes again. Thanks everyone for the help.