@Irene308
They are on their way - if they are on their way from the US, they may be inside an aircraft and not able to use their phone or other devices for communicating.
Did you not ask them for their travel plans, flight number etc ? It is always best to establish these facts well in advance. If they are touring Europe like so many from the US, best to get all info to them and from them before they depart from the US.
The only thing you can do now is try to contact them repeatedly, hoping they will switch their phone on. I always ask my guests to switch their phone on the minute they land so that we can get in touch should there be any travel disruptions. It is also an idea to monitor the airport page to look for delays, but then you need to know their flight number. If you know where they are arriving from, you could do an educated guess. Many long hauls land in the morning, and then you can estimate how long they need to drive from the airport.