Your last session of burining DVD was not completed properly and would have been interrupted/stopped in between. Try two things:
1. Delete/remove all stuff that you placed to be burned in DVD. This will show no stuff pending for ‘burn’. Then copy back the stuff to DVD area and burn it.
2. If point no. 1 does not work, then replace the existing blank DVD with a new DVD, do step 1 again and then burn. It should work.