What Plextor drive do you have? Also, what version of Microstudio?
Microstudio 2.400 supports up to the Plextor 48x drive. If you have the new Plextor DVD burner it is not yet supported.
If you have Microstudio and the Plextor drive both that are supported, then it could be your ASPI drivers. You can reinstall these from:
http://forum.mtu.com/showthread.php?threadid=493
If this still doesn't work, then reinstall Microstudio.