If you've lost your file icon, you can put in another one or find the old one.
First, open up Windows Explorer, then:
Tools > Folder Options
Scroll down to the relevant file extension, in this case,
TXT. Select and click Advanced.
The Edit File Type dialog will pop up.
Click on Change Icon...
If you want to find the system default for it, try this location first (edit as necessary for your Windows directory):
If you would rather another icon, maybe one you've dowloaded, just navigate to it by clicking on Browse... and opening up the folder.
Click OK to save the changes.
The filetype will now show with the changed/reverted icon just selected.