I lost my iphone 11 and someone has turned off find my iphone,and it is no showing on the logged in devices ,how can I regain access to it.i know the imie and serial number

This support article has a list of things to do if your iPhone, iPad, or iPod touch is lost or stolen. --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

- The Find My service described in the article is the only recovery feature Apple offers.

Read this support article to learn what you can do if Find My iPhone or Find My iPad isn't enabled on your missing device --> If Find My iPhone or Find My iPad isn't enabled on your missing device - Apple Support

If Find My iPhone is offline or not working, read this Apple Support article to get help; e.g., when you don't see your device listed, or you see an unexpected device or an alert that says your device is offline. --> If Find My is offline or not working - Apple Support