All my ai. document in one file disappear when I am editing these documents

Hi, I was editing my ai. document for homework, and I go back to the original folder to open another one, and its all disappeared! I search in my computer, and its all in history folder  and naming without ".ai" and all file type and logo were changed. I have no idea what happen. all the disappeared documents were open within two hours before disappeared situation happen. My laptop is game laptop, Win10, no virus. Right now in that folder, there are still two ai. document remain. 

The Chinese warning in picture means "Could not find the document, please check whether the name is correct and try again".