If all your file extensions have changed, it sounds like a virus or malware. Run a good antivirus program ASAP. A BAK file can be opened in Notepad if it’s text based. The question is it depends on the files actual content. Where did the file come from? You could always make a copy and rename it and change the file extension to the program that created it and see if it opens correctly. Better yet, try right clicking the file and select OPEN WITH and pick the program to use to try and read the file.