how do i transfer my cds to itunes

How do I transfer music on my computer to ITunes? With some sort of "update" that occurred with Apple ITunes recently, the ITunes library that came back did not include many CDs and individual songs that I have previously had in the library. These songs were from CDs that I had earlier (insome cases, far earlier) downloaded into ITunes. This was a fairly simple process in the past, but I can't see how you are supposed to do it now. Looks to me like Apple wants me to buy them again.