Dates format suddenly changed to general formats

[Edmon44]

Hello. I was working on my excel file (with multiple sheets), when suddenly, all the dates format changed to general formats.  I started working on this excel file 5 years ago until present and now, all the dates formats became general numbers!!!  I don't know what I clicked!  Can anybody help how to restore them back to 

[#2]

@Edmon44 

Select the cells that you want to change the format to date,

Or Ctrl + 1 , then from the Format window, from the category choose date, then click on the date format you want.