Not dead. Mine does that every once in a while. He will POP back out. I have a bubble tip and 2 long tentacles. They have all done it at least once. I don't know why. Maybe someone else can answer that. My LTA seems to put it's tentacle in it's mouth when I feed it. Maybe to help push it in, I don't know. I just sit and watch it. It facinates me to watch these things.
But to answer your question, No, I wouldn't think it is dead.
125 Gal-35 Gal Wet/Dry-Tetra-Tec PF500 HOB
(3) AuquaClear 301 Powerheads-600 GPH
Automatic Tempature Controller
5-6 Inch Sand Bed-200+ lbs of LR