Assuming that the limit screws are set correctly on the rear derailleur, which they probably are if you haven't fooled with it, and your cables are okay, here is what you do. If you don't have a bike workstand, then get someone to hold the bike up or hang it off the ground with a rope. Turn the cranks and shift to the highest gear (the smallest and outer rear cog). If the chain does not go out to it, then screw the barrel adjustment in the rear derailleur (where the cable goes into it) in until it goes to the outside cog. Beginning with the chain on the outside cog shift to the next lowest gear (keep turning the crank at all times) If the chain does not go to the next smallest cog, then turn the barrel adjustment out until it does. Now try shifting it through all the gears to the inside cog and tweak the adjustment with the barrel adjustment. Also, you might have to take out slack in the inner wire if it has "stretched" out, by loosening the cable bolt on the rear derailleur, pulling the cable tight, then retightening before any of this will work.