Help with downloading Purchased music onto an iTunes.

I got a new computer, and I signed into my iTunes account and downloaded all my purchased music. Then, I took the micro SD card, uploaded all my "non-iStore" music, and put it all on iTunes. But all the music I purchased disappeared. I tried re-downloading it from iStore, but it won't let me log into my account. I need tips on how to get my Purchased music back, and syncing my iPod to it isn't an option because I have a lot of music on there that I didn't have on my phone, and didn't buy from iTunes that I really don't want to lose. Any help is appreciate