Please tell me what I'm meant to do?

Since my diagnosis with cancer, my university have rescheduled my dissertation so that I can hand it in before the exam board. I have not completed my dissertation because I have been too ill from the medication that I have been put on and the cancer has spread, so I'm going to need to have an operation in a few days. My concern is, that I'm so scared of telling my head of year and project supervisor that I still have not completed my dissertation. I don't know what to say to them - it's the end of the academic year, so the only time I can really do it is in the summer or next year. But if I do do it next year, i will have do another project. I'm so scared of them getting angry with me about it :-(. what do I do?