My phone was stolen how do I shut that phone down so my cloud can’t be accessed

Hi there

I am sorry that your iPhone got stolen. First, report the theft to local law enforcement. After that, follow these steps

When you mark your iPhone or iPad as lost, you remotely lock it with a passcode to keep your device and information secure. To help someone return it to you, you can display a custom message with your contact information. When you mark your device as lost, the cards and passes that you use with Apple Pay are also turned off.

• Mark your iPhone as lost on iCloud.com/find
• Mark your iPhone as lost in Find My

If your iPhone or iPad is lost permanently or was stolen

• Change your Apple ID password: Sign in to appleid.apple.com, then choose Sign-in and Security from the sidebar. Select Password and follow the instructions.
• Remotely erase your iPhone or iPad: If you have access to a trusted device or a family member's device, erase your device with Find My. Otherwise, erase your device from iCloud.com/find.
• If your iPhone is covered by AppleCare+ with Theft and Loss, file a claim for an iPhone replacement. Don't remove your iPhone from Find My or your Apple ID until your claim has been fully approved.
• Report it to local law enforcement. They might request the serial number of your iPhone or iPad, which you can find even if you don't have the device. Learn how to find the serial number of your device.
• Contact your wireless carrier to report your missing iPhone, ask them to disable your account, and file a claim if your iPhone is covered under your wireless carrier plan.
• Remove your iPhone or iPad from your trusted devices: Sign in to appleid.apple.com, then choose Devices from the sidebar. Select your device, scroll down, and select Remove from account. If you filed an iPhone Theft and Loss claim, don't remove the iPhone from your trusted devices until your claim has been fully approved.

See this link for further reference

Find your lost iPhone or iPad with Find My - Apple Support