Common problem with shifters is the cable maybe kinked and become chewed up inside the shifter, so its worth trying a new gear cable.you can find technical documents for your gear components on the shimano website http://si.shimano.com
Maybe start here and if you can't figure it out, take it to a Bike Shop. Ther's only so much useful mechanical advice one can remotely lend with no details.
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Common problem with shifters is the cable maybe kinked and become chewed up inside the shifter, so its worth trying a new gear cable.you can find technical documents for your gear components on the shimano website http://si.shimano.com
Maybe start here and if you can't figure it out, take it to a Bike Shop. Ther's only so much useful mechanical advice one can remotely lend with no details.