My 2005 Volvo s40’s CD player isn’t working. It just comes up as “unreadable” for any disc I insert.

 

did you try a CD lens cleaner product? Google around for a product that's recommended for cars vs home stereos or PCs.

Are you using purchased CDs or CDs you burned on your PC? If you burn your own music CDs, you may need to slow the CD burner to the slowest rate to allow the CD writer to make clean "pits".