Odd Characters instead of values
 

Several of my excel files stored on my computer have this problem. Not all
of them, just some of them. When I try to open the file, I get nothing but
character, such as squares or parenthesis repeated in all the cells. The
names on the tabs are still there, just none of the data. They look similar
to something that is in a foreign language that excel does not recognize.
Could this be the work of a virus? How can I recover these files?

Thanks
SteveW