Hey there petroulias,
Thanks for reaching out to Apple Support Communities. We're happy to provide some assistance.
If your device is lost, we recommend following the actions in this helpful resource to help locate your device and protect your data:
If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support
As mentioned in Use the Find My app to locate a missing device or item - Apple Support:
"Mark as Lost remotely locks your missing iPhone, iPad, iPod touch, or Mac with a passcode, keeping your information secure even if your device goes missing. You can also display a custom message with your phone number on the Lock Screen, and keep track of your device's location. Learn more about how to use Mark as Lost."
Changing your Apple ID password will not remove your Apple ID from your lost device.
We hope his helps.
Have a great day!