Okay, lol he's not actually gone and it's not a bug. When you go speak to Cassandra and nothing comes up about cullen, it's because you don't suppose to actually tak to her (Yeah, I know kind of through me in a loop too), you need to go to the Inn on her right. There should be an yellow/grey exclanmation mark in the building on her right (By the staircase and not the 'fast travel'). Once you go in, a cutscene will open and you'll see Cullen and Cassandra arguing [Spoiler]. After that little debate, Cassandra will tell you to talk to him, opening another quest called 'Perseverance'. Now you can return to where he is origanially designated and a cutscene will open with him throwing a box of lyrium at the door as you enter, towards to end of the conversation you have 4 choices, leading to 2 decisions. Which is him starting to take lyrium again or not (Which is mainly dependent on if you chose to side with the templars and not the mages). After the cutscene finishes, you'll complete the quest and boom, he's back lol.
Jay - you're wrong.
Cullen being traped in purgatory due to the Sera pranking quest blocking the messenger flag script is a known bug.
Outside of reloading prior to pranks to trip the flag, there is no legitimate way to reset Cullen currently (unless someone wants to share the command code).
