Check the shifter cable. There is an adjustment at the rear derailleur that you can turn to loosen or tighten the cable tension. Put the chain on the smallest gear on the rear wheel. Shift up to the next gear. If the shift does not happen, turn the cable adjuster to tighten the tension on the cable. If you shift and the chain moves up two gears, loosen the cable. Park Tools is a good website for this sort of adjustment.