Front or rear?
Rear: Flip the bike onto the bars and saddle, then shift into third while turning the crank. Use the barrel adjuster on the back of the derailleur to align the derailleur over the proper cog. If this doesn't work, you may have a bent cog or a bent derailleur hanger. Does it only happen in third? Inspect your third cog for any deviation.
Front: Shift into the small chainring and release the derailleur cable. Pull tight and re-attach. Shift into the big chainring now. If it still won't go, use the barrel adjuster on the shifter co tighten the cable until it does shift. If necessary (because the derailleur will not go to the big chainring on account of a mechanical impedance) adjust the "H" screw to allow the derailleur to move far enough out to shift on to the big ring.
If all else fails, check with the bike shop.
Hope this helps,
matt
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