Music files are gone/missing after update.

I updated I tunes several months ago. I have all my music organized on an external hard drive. After updating, all my music folders are there in order by artist, and the propeties read with how many files are in each, but the only thing that shows up in the files is the album art. I have thousands of songs accumalated over the years, so can anybody help? I know they're still there, but the PC isn't reading them