if you are using login page in your website go to web.config file
<authentication mode="Forms">
<forms loginUrl="login.aspx" defaultUrl="index.aspx" >
</forms>
</authentication>
replace your authentication tag to above (where index.aspx will be your startup page)
and one more thing write this in your web.config file inside
<configuration>
<system.webServer>
<defaultDocument>
<files>
<clear />
<add value="index.aspx" />
</files>
</defaultDocument>
</system.webServer>
<location path="index.aspx">
<system.web>
<authorization>
<allow users="*" />
</authorization>
</system.web>
</location>
</configuration>