Step one, check if it's the shifter or maybe a cable issue. Pull the wire away from the frame like a bowstring, then work the shifter. If you don't feel any cable movement, then it's either a broken shifter, or possibly the cable's head isn't engaged. OTOH, if you feel cable movement, then it's simply a matter of pulling out all the slack, and resetting the cable at the Ft derailleur pinch bolt.
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