Q: What is it with songs that are not from the itunes store, but are coming from cd's and other download sources that all of a sudden do not play on my iphone and have a red stop button in a red round circle around them, behind the title?

Songs that I have had previously downloaded from various other sources, but NOW ALL of a sudden they DO NOT PLAY on my IPHONE and DISPLAY a RED STOP BUTTON in a red round circle next to the title.  Everytime I click on it it DOESNT play.   What happen???? How can i get them to play.,   Why should i have to pay for the songs, and why are they in the ITUNES store and not in my regular playlist.