Act in the first minutes
Speed limits the damage. From another device, open your phone or computer maker's official "find my device" service to locate, lock and display a contact message. Do not travel to a location shown on a map yourself; treat it as information for the police.
Lock the device remotely and sign out of it. Keep the erase option ready if it holds sensitive data.
Call your mobile network to suspend or replace the SIM so it cannot receive your one-time codes or calls.
From a trusted device, change the password for your main email, then mobile money and banking.
Protect the accounts the device could unlock
A signed-in phone can approve sign-ins and read messages. Sign out other sessions on your important accounts, remove the lost device from each account's trusted-device list, and confirm your recovery phone number and email still belong to you. Deny any authentication prompt you did not start.
Decide whether to erase
If the device is unlikely to return, or it holds work, financial or identity documents, erase it remotely. A device protected by a strong screen lock and encryption is far harder to misuse, which is why a lock matters before anything is ever lost. A separate backup means erasing costs you nothing you cannot restore.
Report it
Report a theft to the police and keep the reference, especially if you need it for an insurer or employer. Tell your bank and mobile-money provider if those accounts were reachable from the device. If a work device is lost, tell your employer immediately.
Prepare before it happens again
Set a strong screen lock and turn on device encryption and "find my device".
Keep a current backup so a lost device is an inconvenience, not a loss.
Note your device serial or IMEI (dial *#06# on a phone) and keep it somewhere separate.
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