It sounds like a person who used to have a generic MP3 player tried to put music on an iPod using the same methods as the MP3 Player. As you can see this does not work.
With an iPod you can't just open Windows Explorer or Finder to drag music from your computer and drop it on the iPod, even though it lets you do that by default. The reason is the iPod has 2 modes. USB Drive mode, and iPod mode.
Dragging music or documents using a file manager and dropping them onto the iPod is taking advantage of USB Drive mode. Basically using the iPod like a thumb drive. This was useful years ago when thumb drives were small and files were huge. But the music that is put on this way is not accessible by the iPod. If you have done this, then you need to use a file manager to navigate to the iPod and remove the music that was put on this way. It uses space, but the iPod can't see it.
The correct way to put music on an iPod is through iTunes. The primary assumption is that ALL of your music is in iTunes, if it is not, then copy it to your computer now, and load it into iTunes.
There are 2 modes (technically 3) of use with iTunes. Manual management (drag from the Music section of iTunes and drop onto the iPod) or the Sync mode, where you configure iTunes to put music onto the iPod, and iTunes automatically syncs data between itself and the iPod.
I use the Sync mode which can be done as a full sync (sync all your music) or sync by specified criteria (Playlist, Artist, Album, Genre...the 3rd mode).
Plug your iPod into your computer that has iTunes on it. On the top bar of buttons you should see your iPod appear. Click on it. This will take you to the general settings for the iPod. Near the bottom there will be check boxes for Manually Manage Music and Videos and Enable Disk Use. I personally uncheck both of these. If you plan to use the drag and drop method make sure the Manually Manage setting is checked. Since we all have big thumb drives now, i recommend to uncheck Enable Disk Use. Click Apply. This may prompt you if it is OK to erase the data that is currently on the drive, assuming you have your music in iTunes already, it is OK to delete these files.
Now in the left hand column, under your iPod, click Music. The right side should have changed. Make sure that Sync Music is checked. Then choose whether you want to sync your entire library (all your music needs to be smaller than the iPod storage) or Sync specific Playlists, Artists, Albums and Genres. If you only have 17 GB of music in iTunes, then selecting to sync your entire library is good and click Apply.
Wait for the music to be transferred, you can see the progress in the top center bar of iTunes. Once complete, you can unplug your iPod and find the music and play it.
If you still don't see music on your iPod, let me know and we'll go through a Restore process.