This is a fairly common problem, and has two solutions to it.
First, try resetting the tablet. Occasionally the button becomes unresponsive, and you just have to reset the system and it works again. To Reset the system, you will enter System Settings and then Backup & Reset and you can press on Reset and the system will be resetting to the initial state.
If this does not work, you will have to replace the Digital Board inside your tablet. Follow the Lenovo IdeaTab A2109 Digital Board Replacement Guide.
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