In times’ past, Fiverr CS used to delete unfair reviews - times have changed and this doesn’t appear to be much the case anymore. Even with the option to modify a review, it appears to be a manipulation of feedback in Fiverr’s system.
Rather than dwelling on a 1-star review and its unfairness, consider using your rebuttal feedback to lay out what you did for the client and that no communication on the buyer’s side was given before the 1-star feedback. You can mention that you tried to contact the buyer but was unable to. It’s my thought that a 1-star review will be nothing in the grand scheme of things.
Most buyers are liable to overlook it, especially given the number of positive reviews you have. Of course, how you word your rebuttal will dictate to potential buyers your own demeanor to the situation. If you come across as hateful, they’ll think the feedback was justified. If you come off as professional, they’ll just think of it as the buyer was having a bad day or something of the like.
Don’t bother contacting Fiverr CS - the only thing you’ll be given is a template message about reviews being final and they help to build a fair market - yada yada… something like that. Whether it’s justified or not is not the point in their eyes. They just don’t care.