Music lost; Apple thinks I have a new iTunes library?

When I updated to the newest iTunes, I didn't know I was going to lose all of the music on my computer, but that's exactly what happened. Which means I no longer have copies of:

- Music ripped from my old CDs (I have about 100 CDs, so they take hours to rip)

- Music I purchased from iTunes (This is in the iCloud, which I don't remember signing up for, but at least I have copies of this music)

- Music I purchased from other music stores, because iTunes didn't have it (I bought some stuff from BandCamp after the upgrade, and that's still there, but I have literally lost music that I bought and paid for because of the update, and that's not cool)

- Music that was offered for free from certain indie performers and was not pirated (seriously, if someone gives you music, and you decide you like it, you should be able to keep it)

- ALL of my playlists

But this isn't the worst part; it's a major pain, but I can re-rip my CDs, and get my other music purchases via an old computer. The worst part is that suddenly now, I can't add my new songs to my iPhone, because iTunes now insists that I'm using a completely different iTunes library and has to delete EVERYTHING from my iPhone first, forcing me to start all over in putting music and playlists on my phone. Which, in effect, means that I can't add new songs, because I don't want to lose everything.

Is there any way I can fix this? It'll take me dozens of hours to get everything back onto my computer and phone, and one bad update shouldn't cost me so much time to undo.

My computer is relatively new, and doesn't have a backup yet. My old computer has issues (which is why I got a new one), and is a PITA to use, but is still barely usable. The new computer runs on Windows 8.