Excel 2010-What caused contents of cells to clear on original document?

Without opening the file, I copied a 2011 file to set up a new separate 2012 file. I changed the name of the new file by just deleting 2011 and adding 2012. I cleared the cells in the 2012 and added new information. When I went back later to the 2011 file, the cell contents had been cleared and the new information was also there. The two files have identical information in the cells. All the valuable information for 2011 was gone. Why did this happen? I have done this same procedure for years with no problem.

I have tried to restore the file, but have been unable to do so. I have checked my backups and don't find the right one there. I have searched for information to try to find out what to do. This was information from my business for tax purposes. The restore setting for Excel is turned on. What can I do without spending hours to regather the information?

Thanks for any help you can give me!

CJeanS