shifting problems
 

I have an old Specialized Hardrock that hasn't been used in years. I pulled it out the other night, aired up the tires and it rides fine except when shifting into a lower gear on the rear derailleur it skips two gears. The thumb shifter does not shift at the normal position, but goes almost twice as far as normal. Is this a problem with the shifter or cable, or with the derailleur? I am a novice to bike repair and would prefer not to tamper with the deraileur if not necessary.Shifting is just fine going to a smaller gear.
Thanks,
Brandon

The shifters are probably gummed up from lack of use. There is a hole inside of the shifter buttons were the cable goes in, squirt some lube like Tri flow or WD-40 into the hole and then shift it through the gears till it frees back up. This should fix it but on a rare occasion the shifters might need replacing,New 21 speed Shimano shifters are around $35 and labor .