Yes, reboot, ie, power off and on again. When it turns off after 10 minutes, that's only going into sleep mode.
I can only guess that the media files have ended up in the wrong folder, or that the settings on the phone need changing. Have you opened the phone in My Computer (with File Transfers enabled)? You should be able to see the SD card, and dig down to see the Music folder. If you find that, and the music files are all present and correct, then you may need to go to the settings in whichever music player you have chosen (I recommend Shuttle+ or Poweramp), and select the folder which holds the MP3 files. You will only need to do this once, but sometimes apps are set to default settings, and as the apps are the same for all Android phones, some may need tweaking for whichever phone you have. That said, most would look for music files on the SD card, so it seems a bit odd that they cannot be found.