I opened a duel server (5k), and cause im pretty good at jousting i risked all my 5k i had for one jousting duel .
an opponent came, i was going to win and he leaved. and my 5k was gone.

What should i do ? ? ?

Someone did that to me before and I lost 3k
What should I do too?

guys you cant do anything about that

That's what you get for risking such large amounts of TC on betting servers. You may want to try messaging an smod (blue name) about it, but I doubt anything can be done.