I used to have 8th dan black belt but after i stop playing toribash 1 year go i am having 5th dan black belt -.-

help me how does this happen what is the solution to get 8th dan black belt again

Pretty sure because you were never 8th dan, otherwise you would have gotten the 8th dan achievement, since achievements don't dissapear once you don't match the requirements for it anymore.

Maybe you will get 8th dan by playing 3000 games.

And i am pretty sure i used to have 8th dan this glitch had messed up

Unless you did something that resulted in TSA removing 3k qi from your account you didn't.