Is it safe to refreeze a tuna sandwich?
The larger issue is the storage time that the sandwich has already been subjected to. 5 hours at room temperature is enough time for at least some of the tuna to spend significant time in the "danger zone" (the temperature range from 40 to 140 degrees F where bacteria grow most quickly).

At this point I would not consider the sandwich safe, regardless whether you refreeze it. Refreezing will not render it safe again and I would recommend tossing it.