If the proximity sensor function has been disabled (or is not available in your country), the machine might be in deep sleep mode. Touch and hold the screen for 2 seconds to wake up the machine.
If this didn't help, check if the machine is switched to 'ON' and that the power plug is connected to a working mains power socket.
If this does not resolve the problem, please contact us using the Live Chat in the bottom right or call our Nespresso team on 0808 100 5000.