Why can't I open my .rtf files any more

I use libreoffice writer, wordpad, to write a text, but now for some reason I cannot open that file. why? And how Can make it open?

Right click the file.
Select Open With.
Choose LibreOffice
Click the box that says something about “always use this software”

Computers, or more precisely, Operating Systems (Windows, MacOS, Gnu-Linux), associate document types to applications in order to start the application you want wen you double-click a file of some type.
Your problem is OS specific. Look-up “File associations” or “default applications” in your OS to fix what preferred app you like for RTF.

However you can always open an RTF file from LibreOffice in File-Open, whatever the file association in the OS.

great solution. In windows go to settings–choose default app by file type–RTF–libre office writer