Find my iPhone

Refer to: If Find My is offline or not working - Apple Support

Learn why you might see "Offline," "No location available," or "Location Services Off," and what to do.

Why is it offline?

• Your device is powered off or its battery has run out.

• It's not connected to a known Wi-Fi or data network. 

• If you turned on Send Last Location, more than 24 hours has passed since it connected to a Wi-Fi or data network. 

• You might be in a region where we don't offer this feature due to technical limitations or local law.

Hello,

This would occur for a number of reasons. If you had Location Services, Find My, and Find My Network enabled it may be out of power and/or in a remote location making it unable to connect to any other device in the Find My Network to provide a location from.

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

"Can't locate your device?

• You might not have Find My turned on.
• If you're using the Find My app, you can't see your device's location if more than seven days have passed since its last location was sent to Apple.
• If you're using iCloud.com/find, you can't see your device's location if it's powered off, the battery has run out, or more than 24 hours has passed since its last location was sent to Apple.
• Learn how you can still protect your information."

If your iPhone is found and then charged you will likely see the location update after it turns back on. That should also move your Lost Mode indicator from Pending to Activated.

I hope this helps.