I noticed that the Brasero disk burner doesn't open if I have used it before, and it was still running, but invisible. I have to close it with system monitor and then open it again. I was wondering if there is some bug fix for it.
I have installed Brasero 3.4.1 into Xubuntu 12.04 and everything works well. It could be one of the Brasero plugins, blocking the process by sitting in a hidden dialog. Try following:
completely remove brasero
and you can let it like this and install k3b or install brasero back again
or use Ubuntu Software center
Details in this bug report, along with fix.