Riho obviously was out of voice ( if there is such a term ). Couldn't you guys here it? She was doing all she could to pull out whatever she has left. Just looking at the veins on her neck you could already tell this. Riho probably shouted and laughed too much in the course of the event that her voice just ran out.
Riho does that neck thing A LOT, it really bothers me she always looks like she's straining for the next note. Pro tip Riho, you're supposed to make it look easy, not hard.