src/pages/404.js

import React from 'react'
import SEO from 'src/components/seo'

const NotFoundPage = () => (
  <React.Fragment>
    <SEO title='404: Not found' />
    <h1>NOT FOUND</h1>
    <p>You just hit a route that doesn&#39;t exist... the sadness.</p>
  </React.Fragment>
)

export default NotFoundPage