Use a tracking app or service like Find My iPhone (for iOS devices) or Find My Device (for Android devices) to locate your phone.
Change the passwords for any accounts linked to your phone, such as email, social media, banking, and shopping accounts.
Inform your mobile carrier that your phone has been stolen so that they can deactivate your SIM card.
Contact your local law enforcement agency to report the theft.
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If you can't recover your phone and it contains sensitive information, use remote wipe features provided by services like Find My iPhone or Find My Device to erase all data on the device remotely.
Keep an eye on your accounts for any suspicious activity.