In addition to Vladimir's answer, trace the route of the cable from the shifter to the derailleur, making sure the sections of housing are inserted into the shifter and frame stops. If a section of housing has come loose the cable will not be able to pull the derailleur.
If you have exposed cable (likely along the down tube) then you will also be able to see if the cable is moving in response to the shifter.
If the cable is not moving in response to the shifter a possibility is that the end of the cable has become separated in the shifter.
When you find the problem and fix what ever has broken, you'll definitely need to re-adjust the derailleur as you messed with the limit and indexing adjustment. Follow the Park Tool procedure Vladimir linked to.