Need help with excel file repair

One of my excel file sitting on my online file share corrupted due to some unknown reason. (User claimed that she cannot save the file back after edit. She ended up terminate excel from task manager. When next time she tried to open the excel file, she got the following message.)
“Excel found unreadable content in ‘A.xls’. Do you want to recover the contents of this work book? If you trust the source of this workbook, click Yes.”
I tried to use office 2007 and 2010 to open the file, same error, cannot open the file.
I tried use office 2011 from Mac to open it; I got a different error message:
“A.xls cannot be accessed. The file maybe read-only, or you may be trying to access a read-only location. Or, the server that document is stored on may not be responding.”

I went to the online share server and found there are two files inside the folder.
._A.xls (4KB)
A.xls (32KB)
When I click the 4KB file, I got:
“The file you are trying to open, ‘._A.xks’, is in a different format than specified by the file extensions. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?”
Please advise how to fix/repair this file. Or is there any application I can use to repair the file?
Thank you so much for the time and help.