lost data on dvds

I have several burnt dvds done as data dvds. In going through them for files and programs recently I found many of them are read as being blank or corrupt or unknown format. Has any one else had this problem? What causes this since they have not been subjected to heat cold or magnectic forces. The information that were on them was there and accessable at one time. Any suggestions on getting the data back on the corrupt or unknown format ones?

 

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