Hey Krisan100,
We understand that your iPhone suddenly alerted you when the device was fully charged and your iPhone screen froze prior to charging the device. We'd be concerned too if these issues happened to us, and we'd like to help!
You may have heard one of the 'Government Alerts' that might be enabled. Please check under Settings > Notifications > scroll all the way to the bottom under 'Government Alerts'. Are any of them enabled? If yes, you may have heard one of the alerts from those selections. You have the option to disable any of them if you'd like.
When your iPhone freezes unexpectedly, take a look at this article and follow the steps that are provided: If your iPhone, iPad, or iPod touch won't turn on or is frozen
Let us know if you have any additional questions or concerns.
Warm regards.