Code Inside Page Load Not Executing

I am beginner to .Net development, so i am looking for a favour.

Question 1 :
protected void Page_Load(object sender, EventArgs e)
{
txtUserName.Text = "Focus";
txtUserName.Focus();
}

I am unable to get focus to my textbox

Question 2 :
I made javascript code for client side validation.
function loginjs()
{
if(document.getElementById('txtUserName').value.length==0)
alert("UserName cannot be blank");
else if(document.getElementById('txtPassword').value.length==0)
alert("Password cannot be blank");
}

this code executes but it is not stopping further code execution. I mean it is only alerts but in addition to that i need to stop further execution.