How do I recover missing albums from my iTunes library on multiple devices?

The number of albums in my iTunes library whether on my laptop i-tune, my phone etc has simply disappeared. I had hundreds of albums that appear to have been reduced to 20 odd albums in total. I had some that were on my phone that i could see were still in the cloud but it wouldn't let me download them, and it was asking me to re-buy them from iTunes store.

Also I have back-ups of previous libraries but they will not load to view the files.

[Re-Titled by Moderator]