The gear system of my bike stopped all of a sudden by itself...now I'm unable to change the gear in my bike...It's till in 1st gear...What should I do to bring back the gear system to normal working condition...Can I know the reason for this behav

The gear system of my bike stopped all of a sudden by itself…now I’m unable to change the gear in my bike…It’s till in 1st gear…What should I do to bring back the gear system to normal working condition…Can I know the reason for this behaviour so that I can avoid this problem in future

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