Can read CD/DVDs but can't write

Dimension 8400, with 2 drives, which worked fine for years.  Then tech support worked on it by taking control of it online.  Afterwards, I found that I my DVD write drive wouldn't write anymore, although it reads.  When I tried to write, the computer just didn't recoginze that an empty disk is loaded in the drive.

I called tech support.  They again took control of my computer online, said something had been turned off, and they fixed it.  I did a test while I was on the phone with them and itt worked; I wrote to a disk. 

But the first time I tried to write to a disk after the "fix"  (which was a few months later) I found that it again wouldn't write or recoginze that there was a clean disk loaded.  It will still read.  Of course, by then, my support contract ran out. 

Does anyone know what I need do to fix this?