.xls not recognised as excel anymore
 

I have 3 Excel files, 2 .xls and one .xlsx protected with a password, saved on the same day.
Previously they opened without any difficulties. Other excel files saved on earlier/later dates all work normally. Only the 3 i saved that day don't work anymore.
When i want to open them now, excel doesn't recognise them as an excel file anymore, and wants to import text. Result: only some strange à#AÄ@ things instead of seperate sheets with formulas and numbers...

Is there a possible way to recover these files as a regular Excel-file?
I would REALLY like to have especially the .xlsx file back as it was!!