Missing a defense is the same as registering for a single exam. If you miss the self-study exam, just register next time, and the defense is the same. Of course, you have to re-register the registration fee and other related fees. After the registration fee is paid, the self-examination office will tell you the specific operation process of thesis defense and follow the instructions. I remember that when I submitted my thesis, the Self-taught Examination Office sent me a written process to the effect that the first draft of the thesis must be submitted before a certain date, when and where to reply, and so on.