Cannot Rewrite DVD-RW on Mac OSX Leopard

I bought a Imation DVD-RW some time ago and used it to store my videos. Then today when I tried to delete the videos an error occurred. Then I checked the Get Info and saw that the disc was now read only. :sad::sad: Now I cannot reuse the disc!
I tried to repair permissions in Disk Utility but the button was blocked. :a:a:a:a