did you find a small spring than goes over the barrel ? and the two parts goes into the plastic adjuster. you will have to remove the cable from the front dérailleur, I think you might need to replace the barrel adjuster parts which you can buy at your lbs for around $5. strew the adjuster back into it's post back off by one turn run the cable though it, reconnect it to the dérailleur and adjust the tension so it will shift again using the adjustment barrel.