Check your customerrors mode setting in Web config
By default it will be
<configuration>
<system.web>
<customErrors mode="Off"/>
</system.web>
</configuration>
This setting will show actual error on runtime
<configuration>
<system.web>
<customErrors mode="On"/>
</system.web>
</configuration>
This setting will not show actual error on runtime
<configuration>
<system.web>
<customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
</system.web>
</configuration>
In this case you can also set redirect to default error page.
for this check this link