my iPhone was stolen.how can I switch on my devices when is of

Hello ~ Take a read through this link to better understand using find my and follow the advice given by * Lawrence Finch* in his post.

Use the Find My app to locate a missing device or item - Apple ...

~Katana-San~

What devices do you want to switch on? Any devices in your possession you just turn on. If you mean the stolen phone there is no way that you can turn it on if it is off. Did the phone have Find My enabled before it was stolen? If not, there is no way to track it. If Find My was enabled you can track it when it connects to a cellular or Wi-Fi network.

But either way, the very first think you should do is See→If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

See→If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support