So, I read about this problem a lot online, but didn't find any solution. Most recommended firmware updates, but my problem was way easier to solve.
Basically, my computer couldn't read DVDs, and was unable to finish writing DVDs also. Both worked fine with CDs though.
How did I do it? I changed the region on my DVD from 2 to 1 in its properties settings. Suddenly all DVDs came back to life.
So, the laser isn't ruined, the firmware isn't bust. The problem was the way windows and Nec handle regions. Definitely something that ought to be fixed asap.
Hope others in my situation will find this thread via Google, and that it works for you too.