My DVD-CD RW is not working as writer now ,There were some DVDs and CDs written and others not ,but now it doesn't write and DVD or CD (My laptop is TOSHIBA L300)

Try using a slower burn mode. This happened to me and my Dell laptop kept rejecting burning on discs. I turned down the burn speed quite a bit and everything worked fine..

You may want to reinstall the drivers for the burner as well.