Suddenly most of the files and folders in my sd card are now named "FSCKxxxx.REN". How to fix this without root?

^{Summing up from the comments:}

The most likely cause is your SD card being in the process of "breaking down": When you connected it to your receiver, it detected problems and ran fsck (the File System ChecK utility) to repair it – which found issues with some files and directories, couldn't figure their names, and thus "saved" what it found using those "cryptic names" (the FSCK prefix seems to confirm this, though normally such "recovered blocks" are moved into a LOST+FOUND folder).

After having connected the card to your PC using a card reader, the PC again found the card faulty and had to repair things. Of course it couldn't restore the FSCK* files and directories to their original name, as that name was lost. As you correctly discovered, using the file command you can determine the file type, rename the files manually, and many, most or (if you're very lucky) even all or them might be intact.

So here's what you should do:

1. Back up the entire contents of the card


2. Get yourself a new card ASAP
   
   (until you did, repeat the previous step regularly; it might very soon start losing data)


3. Copy the backup to the new card, and use that

And next to that, go the described way to identify the FSCK* files, which you then can rename to "useful names" and also copy back to the new card.