Lost message notifications after 13.3 upgrade

Do any other notifications work? If all you have done is restarted the iPhone, then you can try a force restart. See this, Force restart iPhone - Apple Support

After that, if it still won't work, and you have checked to make sure that message notifications are turned on, a sound is set, and you do not have any messages with Hide Alerts turned on, create a current backup of the device and then restore the iPhone to that backup. If you have not updated to the latest iOS 13.3.1, then the iPhone will update to that iOS and then restore your backup. Test after that.

Thank you for the information that you had already updated. Now, did you try creating a current backup and then restoring to that backup. That is the next step for troubleshooting.

What troubleshooting have you tried? Are you connected to any external device, such as a Bluetooth speaker, an Apple Watch?

You're welcome.