Lost questline

Hi,

While questing in Hellfire Peninsula, I have unknowingly abandoned an important quest in a questline and I am now unable to access the reputation for Mag'har and the mounts they offer.
Is it possible to somehow recover the questline by allowing my character to reattempt the first quest?

I would have written a ticket instead of posting here but the only category I can choose is a "Ban appeal".
Any help would be greatly appreciated!!!

Server: Icecrown
Character name: Rezune
1st quest of the questline: The Assassin(id: 9400)

Think the trainer and/or mount vendor are part of Orgrimmar, so Maghar rep may not be needed, at least for both.

Go through the quest NPC's on the questline, one should have the quest icon.
https://www.wowhead.com/quest=9400/the-assassin