Molten Vault Arena lockout

So fought the forgemaster and we both killed each other at the end. When I revived I could not get back down to the arena with the quest item so essentially I am locked out of completing the quest.

Have you tried recreating the instance and killing the boss again?

Edit: After actually resetting the instance the boss did respawn and I was able to finish the quest. Thank you.

When I reset the instance the boss is still dead and it tells me to pick up the hammer, but I cant pick it up because its in the Arena below and the elevator that took me down the first time is not available

Happened to me as well. I died first and initially was just a little annoyed at whatever one-shot me when the boss was on the brink of defeat, but then somehow the boss did eventually die a few seconds later (probably delayed damage of some sort). I was prepared to lose the loot, but I was not expecting the arena to become inaccessible for the quest drop after witnessing the death animation again.