My old Cds are ruined. Why?

Unfortunately you didn't leave nearly enough information to answer your question. What happened is that your CD delaminated. Why it did so could be due to a variety of causes ranging from a defective CD, to inproper storage. Take a look at the URL I've provided and see what rules you broke. Also, if you intend on storing data for an extended time, use CD-R or DVD-R for your storage. They last longer than CD-RW or DVD-RW.