Can't transfer iTunes music from external backup drive to new pc with Windows 8.1.

I transferred iTunes music from my old pc to an external backup drive. I tried to transfer the music back to my new Windows 8.1 PC, but it didn't work (song names showed up in new PC iTunes library, but no music files (e.g.mp3 or mp4 files transferred. I also didn't know how to transfer music from the external backup drive to iCloud. I can play music from backup drive when backup is connected to new PC, but double clicking on a song name from new PC's iTunes program gets me prompt saying "can't locate file" - and nothing plays. How do I transfer music in external backup drive to new PC? Also, do I have to uninstall iTunes on new PC and then reinstall it before I try to do the backup transfer again? Appreciate any help you can give me because I now have no music to play on new PC, iPhone 5 and iCloud too.