Hello! I was confused by my problem .When reopen my mathematica file, all the content is converted to same style, making my file unusable.
Fisrt picture shows what the file should be.
Than when i close the file and reopen it,it changed to be like picture 2.
The file tells me that all these codes are still "input" style, but all my sections are combined to one, and all "In" and "Out" separate my codes.

I think it is because I changed some settings, but getting all things back to default didn't do any change. How can I solve the problem?