Or if cable is still attached to shifter but shifter won't function, loosen bolt where cable connects to RD, pull more cable to position rd mid-cassette, and tighten up. You can also do alot by tightening the snot out of every barrel adjuster to move it up cassette.

Last week, in a pilot run of my new bike, I lost front shifting about 12 miles in to a sixty mile ride. The group takes a windy path, so I left it on the small ring and took the hilliest, shortest route possible to the mid-point coffee stop, and ended up not too far behind the posse. Then put it in the big ring for the mostly downhill return group pounding. Had to mash at points but was much happier on the big ring than the small.