Q: about 30 out of 80 songs that I purchased play on the computer the 80,the orignal file can not be found so they are unable to play

I purchased about 80 songs from i tunes eveything was working great,then all of a sudden about 50 of the purchased songs would not play ,a box came up stating that the orignal can not be found and if I wanted to search for it.What do I do now ?