DVD/CD-RW no longer reading cd's HELP

Hopefully somone can help me I'm pretty computer CLUELESS. I have been adding files to my iTunes from my DVD/CD-RW external hardrive and I went through 6 cd's and when I put in the 7th the e:/ drive was no longer able to read the cd's, kept giving the please enter disk into drive error. Does anyone knwo what might have caused this or what I can do to solve it?

thanks!!!! :eek: