In order to implement it you need follow below process:
Step1: Declare a javascript function which will check session is NULL.
Step2: Call that javascript function with 1 minute interval to to check session is exist? You can use setTimeout() function to call the method.
Step3: If session is null then redirect to login page or display an alert message.
Please follow below link how it can be implemented:
Alert Session Time out in ASP.NET[
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