This happened to me too yesterday. I don't remember when I lost him, but when I went to continue my story he was gone. It acts like he is there, but I cannot dismiss him. Both him and the UI are nowhere to be found. I had a scene with him when he was missing and he was not where I was. He was "out of the game" with a grey like background when he talked. [update]: I used the holocommunicator I received for participating in the Dark vs Light event, which summoned a new companion. That caused the UI to reappear. I was then able to re-summon my glitched companion.

I am having issues with Skadge as well. Along with Risha and Akavi. Both characters have finished their respective class story, but I cannot finish the companion quests on three companions. I was able to talk to Risha and Skadge a little, but at some point they stopped talking to me. I have not noticed it with any other companions (I had already unlocked many, including Blizz). I hope they fix this.