Turn the bike upside down and try shifting through all the rear cogs on each of the different front sprockets. Then tell us which gears it doesn't reach. If it goes to every rear cogs on the outer chainrings but not the inner one, that would be rather odd. If it simply doesn't reach the inner rear cog on any chainring, then either you need to take slack out of the cable (it has stretched) or adjust the inner stop screw on your rear derailleur. To take out cable slack, shift the derailleur to the outermost rear cog, loosen the allen screw on the rear derailleur which holds the cable, stretch it with pliars, and retighten the screw.