Message=Incorrect syntax near 'Userdata'

SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["RegistrationConnectionString"].ConnectionString);
conn.Open();
string checkUser = " select count(*) form Userdata where Username='" + TextBoxUN.Text + "' ";
SqlCommand cmd = new SqlCommand(checkUser,conn);

System.Data.SqlClient.SqlException was unhandled by user code
  HResult=-2146232060
  Message=Incorrect syntax near 'Userdata' 

int temp= Convert.ToInt32(cmd.ExecuteScalar().ToString()); // ***

if (temp==1)
{
   Response.Write("User Already Exists");
}
conn.Close();