with events you should set AutoPostBack to True then use this event :
protected void btnSubmitForm_Click(object sender, EventArgs e)
{
if(textbox1.Text=="")
{
}
}
then check if it is blank ...
you can use this too :
<asp:requiredfieldvalidator id="RequiredFieldValidator1" runat="server" controltovalidate="txtName" xmlns:asp="#unknown">
ErrorMessage="Enter your name!"
ForeColor="Red">
</asp:requiredfieldvalidator>