Some of my original files cannot be located. How do I find my previous purchases?

When trying to open music files, I get a window stating "original files cannot be located, would you like to locate them". When I try to locate them, I cannot find them. Someone told me I might need to restore all my music again, but that Apple only allows this one time. I paid for all these songs, and would really like to be able to access them!!!!!!! Help!!