I had exactly the same problem with my combo drive (which was a Sony DVD/ CD writer, rewriter dual layer + - format drive). I eventually bought a replacement drive but was told that the old one could be fixed. The most likely cause being that the laser had been knocked out of alignment. To re-align the laser requires the use of an oscilloscope. The problem is trying to find someone to undertake the repair, as many independent high street PC repair shops deem it to be "economically unviable". (One quoted me an hours work for around £20 but said it was not worth his while).
David Hughes, February 2008