How did they give you the file, on removable media or via e-mail ? It could be the USB is damaged. If sent via e-mail, is there any encryption being done ? It could be a number of possibilities. You can always try the repair tool.
Click File > Open.
Click the location and folder that contains
the corrupted workbook.
In the Open dialog box,
select the corrupted workbook.
Click the arrow next to the Open button,
and then click Open and Repair.
To recover as much of the workbook data
as possible, pick Repair.
If Repair isn’t able to recover your data,
pick Extract Data to extract values and formulas from the workbook.
If neither automatic nor manual repair works, there are some
recovery methods that might help salvage your data.
You can also take some preventive measures to avoid losing
your work by automatically saving a backup copy of your
workbook or creating a recovery file from time to time.