Stylesheet gets distorted after saving the notebook

Questions
1

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.

1.png
1.png2043×557 120 KB

Than when i close the file and reopen it,it changed to be like picture 2.

2.png
2.png1053×1218 90.9 KB

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?

2

I would suggest attaching the affected file here so readers can have a look.

3

All file were affected

plot.nb (830 Bytes)

4

The contents of the supplied file is plaintext, not a Notebook expression that the notebook file format requires. How was this file saved originally?