The iboat transponder is battery powered, self-contained and very water resistant. They are the size of a small notebook.
One possibility is that the transponder was lost overboard and it is drifting. It can only transmit when it is face-up, so it could have flipped over in a wave and stopped transmitting. That would explain why it didn’t transmit for several hours today. It is transmitting now, so hopefully CG can find it.