If you use ASP.NET login control in your website, by default login session of your accounts expired after 20 minutes. For increasing session timeout there are some different methods but the simpleset methos is change your website configuration in web.config file. you have to add below data in your web.config file for this reason:
<sessionState mode ="InProc" timeout="540"/>
<httpRuntime executionTimeout="999999"/>
but If you are using Asp.net login control your session timeout problem exists after these changes. for increasing session timeout of asp.net login control change authentication tag in your web.config to below tag:
<authentication mode="Forms"> <forms timeout="540" slidingExpiration="true" loginUrl="~/Login.aspx" />
</authentication>
Your session expiration time is now 9 hours(540 minutes)! 
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