Welcome to Android Central! Which mp3 app? If it's an app that claims to give you "free" mp3's that you would otherwise have to pay for, then it's a shady app with shady sources. It may have lost the ability to download the songs you're looking for, because it basically tries to find servers where copies of these songs are (probably illegally) saved. Access to those songs can easily be lost if the server is shut down by authorities, or if the rightful owner of that song makes a legal request to make that song unavailable. Don't rely on these apps to get music -- obtain it in a more legitimate way (i.e., buy the album, or subscribe to a music service).