<!-- Web.Config Configuration File -->

<configuration>
    <system.web>
        <customErrors mode="Off"/>
    </system.web>
</configuration>

I did what it suggest I do, but still the site doesn't work, I mean I would like to have an Idea where to start troubleshooting.

<!-- Web.Config Configuration File -->

<configuration>
    <system.web>
        <customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
    </system.web>
</configuration>

Which of those two web.config examples is yours?

Setting customErrors mode="Off" will allow you to see detailed errors.  However, if the error is with the web.config, then you may not see very much detail.

Also, set the Debug="true":

<system.web>

<pages>

<compilation debug="true">

</pages>

</system.web>