What steps are necessary to reverse other than honorable discharge through a congressman in order to rejoin?

I was discharged under RE code 4 other than honorable for a failed drug test, however, til this day i do not know why/ where it was possibel for me to have come in contact with a drug (cocaine) as i have never used a drug in my life. Upon my charge and court martial Navy Legal on board my ship USS Ronald Reagan while at sea, was not helpful and even stated 'even if you didnt do the drug, your still better off just sayin you did you probably wont be discharged' , well you see where i am now... did not work out so well. I have spent the last year or so in anguish over this as the military is my lifes pion, and now i desperately need help in seeking a way back into the service, i have many many outstanding character references and have never been in ANY trouble in my entire life, any help would be gratefully appreciated. Thank you.