You can try to adjust the barrel adjuster at the shifter. It is located where the cable comes out and will have little nubs on it. Turn it counter clock wise until you can shift to the largest chain ring. If that doesn't work you can pull on the derailleur to see it it will shift to the higher chain ring. If it does you will have to readjust the cable clamp. If it doesn't you need to adjust the high limit screw.
http://bicycletutor.com/adjust-front-derailer/