Without Safe master password, no person can get access to your data encrypted with a strong 256-bit Advanced Encryption Standard (AES) on your devices and in a cloud. More about Data security in Safe.
In addition:
Protect your cloud Deny cloud access for the lost phone as per instructions from your cloud service provider (e.g. for Google Drive, Dropbox, OneDrive, iCloud).
Lock your phone Remotely lock your device (use instructions from your device and operating system manufacturers, e.g. for Android, Apple).