My Laptop Won't Recognize my MP3

Hey guys, I 'm having a problem that I really need some help with. I have a new Mp3 player (I forgot what it was called) and for a while I was able to easily plug it into my laptop or my desktop and it would easily charge and let me download music. But now, it won't let me download music. It charges just fine, though.
If you have a solution to this problem, I would be ever so grateful.
Thank you!