Last night June 7th, while playing the 'find Heydon' quest on hard with my new lvl2 rogue/lvl1 druid my companion died. (I received a "you are no longer connected..." message, but it came and went so quickly during the final battle that I didn't pay it enough attention)
Long story short before I noticed that my wolf companion was at -10 health I recalled out of the quest. After that no more companion. You can't cast revive because the system says "it isn't dead" and you can't summon another because you already have one summoned??? It's kind of frustrating but logging off and back on fixed it though.
Going to try to repeat this today.