Why does Find my iPhone say offline?

I lost my phone today and I immediately got on find my iPhone from a different device by signing in with my apple ID. It showed me where it was at, we went looking for it and it wasn't there which means someone probably stole it. Well it has been hours since that and now find my iPhone says it can't find my phone because it is offline. I don't think the phone is off because when we call it rings. Why is it saying it is offline?

I bet black guy stole it

It may be possible that the person who stole it turned off mobile data, which will prevent FMI from locating it. It can still receive calls, but it can't use mobile data.

Internet turned off. That defeats the find app.

You lost the phone, it wasn't stolen. If someone found it, they more likely than not turned it off. Once they got to where they were going, they popped your SIM card out and put theirs in to see if it worked. If you put a password on the phone, they found that they are locked out of the phone. If not, they turned off cellular data, and are not using Wi-Fi, until they can figure out how to make the phone theirs.

What you need to do is call your carrier, report the phone lost. Log into your iCloud account and list it as lost, so that it locks the phone the second they connect with Wi-Fi/cellular data (with your SIM card in it.)