How to re-connect a ProBeats AirPods once they get disconnected?

Assuming you are referring to PowerBeats Pro this article should help:

Set up and use your Powerbeats Pro with an iPhone, Mac, or other device - Apple Support

Once connected to your device your Powerbeats Pro automatically pair with your other devices that are signed in to iCloud and using iOS 11 or later, macOS Sierra or later, or watchOS 3 or later.

Pair with iPhone, iPad, or iPod touch

1. Make sure that you turned on Bluetooth on your iPhone, iPad, or iPod touch.
2. Place the Powerbeats Pro earbuds in the case. 
3. With the lid open, hold the Powerbeats Pro within 2 inches of your unlocked iOS device.
4. Follow the instructions that appear on your iOS device. If you don't see any instructions, follow the steps to connect to another device.

Pair with a Mac, Android device, or other device

1. Make sure that you turned on Bluetooth on your Mac, Android device, or other device.
2. Place the Powerbeats Pro earbuds in the case. Leave the case lid open.
3. Press and hold the system button until the LED blinks.
4. Open the Bluetooth menu on your Mac, Android device, or other device. For example, on your Mac, choose Apple () menu > System Preferences, then click Bluetooth.
5. In the list of discovered Bluetooth devices, tap or click your earphones.

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If you are instead referring to AirPods Pro the below info will be relevant for those:

Pair AirPods Pro – Apple Support (AU)

Pair AirPods Pro with iPhone, iPad, or iPod touch

Before you begin, make sure your device has the latest version of iOS or iPadOS installed.

1. On your device, go to Settings  > Bluetooth, then turn on Bluetooth.
2. Open the case with your AirPods inside, then hold it next to your device.
3. Follow the onscreen instructions, then tap Done.