Over the years songs have disappeared, even a whole album

Are they still in your country's store ? If they are then check to see if you've hidden them from the Purchased section for redownloading : iTunes Store: Hiding and unhiding purchases - Apple Support

If the rights-holder has removed them from your country's store then you won't be able to redownload them. Do you have them on your backup of your downloads/library, or on an iOS device or iPod that you can copy them back from ?

If you connect it to your computer's iTunes and do File > Devices > Transfer Purchases then they should be copied back to your iTunes library