The first thing you need to do is sort out the PC's failing hard drive because until you do, the problem will more than likely become worse.
In fact, you need to confirm that the reason the music is now missing from iTunes is due to a failing hard drive and not due to the files for those missing songs being deleted. Only once you have confirmed the reason for the missing files will you be able to move forward. Thinking that the hard drive is failing is not confirmation.
In your iTunes Library do the missing songs have exclamation marks next to their title when you try to play them?
- if there are exclamation marks, that suggests that the file for that song cannot be found by iTunes, either because it has been deleted, renamed or moved (or the file path has been changed), or that the hard drive may be failing.
- if the song is no longer listed by iTunes, that will be due to a totally different issue (and not due to a hard drive failure)
It's a crucially important difference.
Check the hard drive to see if you can locate the missing music's files. You may be able to get iTunes to tell you the folder it expects to find the file in. If you find them, can you add them back to iTunes? If so, that suggests brings into question whether the hard drive is failing or is there another reason why the files for that music might be missing? In other words, have you been performing housekeeping and deleted files from your computer using Windows Explorer. (Don't forget, a song listed in iTunes is not actually located "in iTunes". It is listed by iTunes, which then looks in the location on your computer where that file is stored. But if you use Windows Explorer to delete that file, iTunes will still list the song but will no longer be able to play it.)
It's probably why there is now music missing from the iPod. A sync of the iPod with iTunes has probably noted the songs now missing from the library and therefore effectively removed them from the iPod, causing that space on the iPod to become Other. In other words, media that is now unreadable by the iPod. Do not attempt to do another Sync of the iPod with its iTunes Library. If you do, the problem will become worse.
If you do not have any backup of your music, make a backup now (onto an external drive) of what is left. You may need to seek help from a computer repair shop.
If you need to replace the hard drive, you will need to re-install iTunes and add back all the music to your iTunes Library. To do that use the backup. If you do not have all the music baked up you will need to obtain new copies and how you do that will depend on where the music came from originally:
- if the music was an online purchase from the iTunes Store, Amazon, Bandcamp or another online retailer, go to your account with that store ad re-download the music
- for music obtained online form artist websites etc. go back to that website for another download
- if the music was copied from CDs, you will need to make new copies from those CDs
