Accepted paper at conference but not presented
There are two possible cases here:


(Default case) Your paper has not been included to the conference proceedings
volume. In this case, your paper is unpublished. It might still be possible to cite it, e.g. by uploading it to arXiv and referencing it as an arXiv preprint, but this citation would have to leave out the name of the  conference.
Despite your absence at the conference, the paper has been included
to the conference proceedings volume. In this case, your paper counts
as published and you can cite it.