For events, you gotta kill all the enemies the event spawned (but no new enemies should be spawning). You'll still get the background music, etc, even if there's only one single goblin/martian/whatever that's hidden in some unlit cave or something. Easiest way to deal with this is to Save&Exit and go back in (don't leave anything on the ground obviously).
Terraria Community Forums
Xylia
Shyguy
Alright, so I killed all the enemies from the event, which did cause the music to return to normal. The problem is that the event is still occurring even after the enemies have been defeated. So the background is still filled with UFO's, though the music is normal and no new mobs are spawning. Cont.
Shyguy
This is causing Town NPC spawns to stop working as well as the Cultists. Save&Exit sadly does nothing to fix this problem. Seems like the game thinks the event has ended and is occurring at the same time. Any Ideas??
Shyguy
Posting in Bug Section, will see if the rest of the community has any ideas. 