My guess, and this really is a guess, is that your front shift cable is too loose or has slipped in the front derailleur's pinch bolt. Shift the front shifter into the granny (lowest gear) position several times to assure it's at the slackest. Check and see if the cable is very loose and has a lot of slack. If that's the case, loosen the pinch bolt, pull out all of the slack and tighten the pinch bolt firmly. If you are using a teflon coated cable, be sure to scrape off all of the coating where the pinch bolt grabs it.