The cable is probably just stretched and needs tightening. Turn the barrel adjuster anti-clockwise half a turn or so and see if that helps. If it doesn't, tighten it more. If it still doesn't, then it may have slipped at the mech cable clamp. Release the cable, reset the barrel adjusters, and then re-attach and re-adjust the cable.
The cable is probably just stretched and needs tightening. Turn the barrel adjuster anti-clockwise half a turn or so and see if that helps. If it doesn't, tighten it more. If it still doesn't, then it may have slipped at the mech cable clamp. Release the cable, reset the barrel adjusters, and then re-attach and re-adjust the cable.