The majority of reasons regarding this issue are addressed in the Knowledge Base article You have a general issue burning your disc.
Aside from media or hardware related issues, this error may also occur if your files are too large for the target media. Users with MyDVD 6.x or Easy Media Creator 8 and higher have the ability to use the "Fit to Disc" option. This will ensure that your projects will fit conveniently to the target media, thus eliminating the issue.
Data project users should try removing portions of their project to see if the button re-activates.