Out of the blue my iPhone x fell out. I can't restart it. What can I do?

When you say "fell out", I'm guessing that the device has stopped, or are you referring to it falling out of a case, or what exactly do you mean? It may be a phrase that is something said in your local area, but it can mean different things here, since this is an international forum. But, I'll take a guess.

To perform a force restart on the iPhone X, follow the steps listed for the iPhone X in this support document. If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

If your screen is black or frozen, you might need to force restart your device. A force restart won't erase the content on your device. You can force restart your device even if the screen is black or the buttons aren't responding. Follow these steps:

On an iPhone X, iPhone 8, or iPhone 8 Plus: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo.