Emergency Mode

Lost or stolen iPhone

Your iPhone is missing. Protect the accounts it can reach, then decide about the device itself.

What this assumes

You are using a different device: a computer, a tablet, or someone else's phone.
You have internet access on that other device.

Containment. Minutes matter here and nothing else does.

Sign in to iCloud on another device, open Find Devices, select your iPhone and turn on Mark As Lost. This locks it and shows a contact message. Official page
Enter a contact number that is not the missing phone.
Call your carrier and ask them to suspend the line, so the number cannot receive your verification codes.
Change your Apple Account password from the other device.
Change your primary email password next, then any financial account you can reach.

When the immediate situation is over, a plan is what keeps it from happening the same way twice. That needs an account; this page never will.