Ive just completed the lights out quest and serena is stuck inside broken oar grotto. Everytime i leave she stays inside.. anyone know what to do?

Reload a save or wait a few days.

PS3 PC Reanimated, thralls and followers may not exit with the player at this location. If possible, leave your enthralled beings out, at least until it's fixed or a way to bring your thrall out is found. Confirmed for XBOX as well as of Dawnguard.

• PC ?Fix: http://elderscrolls.wikia.com/wiki/Barbas, reanimated or thralls, or other followers may get stuck in some cells (locations) and may not go outside with the player. To fix that go back inside and find the follower/thrall in question, open the console and target the individual who is stuck (you should see a number RefID). Do not click on anything else and close the console. Next, go outside and open the console again to issue the command: moveto player. Now close the console. Your thrall or follower should be standing next to you outside the grotto.
  
• Xbox Fix: For followers, exit the cave without dismissing or asking the follower to wait, then fast travel to Septimus Signus outpost. Wait there 24 hours. Fast travel back to Broken Oar Grotto. The follower will now be standing outside the cave.

This is a problem with several dungeons in the game; mainly Broken Oar Grotto and Shalidor's Maze. The best solution that I found, was not to dismiss them; but, instead, to fast travel to a new location; they didn't always appear there; so, I fast travel back to the dungeon location and *POOF* they appear out of thin air. Make sure you keep them on follow or it won't work. Waiting helps also, as it forces them to respawn near you; but, fast travel first.

Thanks ill try that.