Why can't I find my iPhone?

If you are using Find My.....and the message indicates that the iPhone is "Offline".......there are several reasons for this:

• The the battery has run down
• The iPhone was powered off
• The iPhone does not have an Internet connection where it is located at this time
• A thief has wrapped aluminum foil around the phone to prevent the phone from either sending or receiving a signal. Placing the phone in a Faraday Bag accomplishes the same thing.

Unfortunately, there is nothing that you can do about this except keep checking a few times each day to see if the status of the phone has changed

If you are using Find My and a message of "No Device Found" appears, this indicates that Find My was not enabled on the phone. In this case, the only way to locate the phone will be to physically look for it with your eyes.

More details here: How to find your lost iPhone or iPad - Apple Support

What they said, except look in the car or down the sofa.

There’s fairly lengthy list of possible reasons.

If you might share some of the actual circumstances, we could better assist.

In the interim, see if this outline might help:

How to find your lost iPhone or iPad - Apple Support