Where have my playlists gone?

For some reason, when I opened iTunes 2 days ago it asked for my apple ID. I accidentally clicked on an old apple ID and followed the authentication process. Then I realised my mistake and signed out, then signing in with my current appleID, again following the authentication process. However, since then ALL the playlists on iTunes have disappeared. Also, the whole library has disappeared except some purchased songs. All that's left are songs I've bought/downloaded in the past year. If I sign out and sign back in again with the correct ID the same happens. I've turned the device off and on. I've shut iTunes down and started back up again. (And lots more besides! SO much time wasted trying to get these playlists back!) Nothing. It's just not recognising my ID.

I have the playlists on my iPod, but I don't want to sync to the PC in case it wipes them off there.

I can reupload the songs quite easily and will probably do so, but I want to know why this has happened?!? Where has all the music gone?

This is not the first time I've lost my whole iTunes library before. It's excruciating! Please help!