I accidentally exited the arena during my mastery quest. Now the mission's disappeared and clicking the staff (my mastery item) gives me a red message and I can't seem to be able to re-start the quest. Am I doomed to not have a mastery class? How can I fix this?

You need to abandon the quest in your quest log

That's just it, for some reason my quest log is completely empty.

Did you press the "J" key to check?

Ah OK. I'll look through the missions I have there. Thanks a bunch.

Usually when we talk about quest log, we mean when you press "J" to open up the log. The quest tracker is when you have quests that are on the right side of your screen.

OK I've dropped every quest I had in the log (by using J) and the item still gives me no quest. The first Indian doesn't either nor does Korin. :(

I did the same thing at the same area of the mastery quest that you did, I was able to abandon the quest and retake it from the same NPC who sent me to the arena. Try relogging, other then that post some screenshots if it still isn't fixed after all that stuff.

And why did you remove all your quests? I don't know if you are aware of this but you could have seen which quest it was by clicking on them and seeing which quest had the Mastery Weapon/Skill in it as a reward.

Awesome advice guys thank you. I went back to the NPC who sent me to the arena, got the quest, and got my mastery.

I removed all quests to be thorough. I don't mind going back to the starting area and getting them all again slowly and painfully if it was gonna help me fix a broken character.

Once again, thanks guys!