I honestly don't know how to reproduce this sadly.  I tend to use quick save quite a bit.. so sadly after trying to fight Maerwald and failing I reloaded and my wolf pet has disappeared.  I can see my wolf's circle briefly before it flies off the screen but his portrait is now gone and so is my wolf.  If I enter combat now and die the game simply sits there since they can no longer find the wolf to fight and being a ranger, I've pretty much lost my main companion.  

My Ranger Pet Bear disappeared as well, i think it is a bug the developers know about.

All we can do is sit and wait for the patch to come.