BLYNK
BLYNK.IO       📲 GETTING STARTED       📗 DOCS       ❓HELP CENTER       👉 SKETCH BUILDER

Connection

I need that when losing the internet connection, that is, when losing the connection with the BLYNK servers, some motion sensors are monitored and if it is the case of any activation they turn on a buzzer

Add details
• Arduino Mega with Ethernet Shield
• Smartphone OS (iOS or Android)
• Blynk server

You need to write your code in such a way as to keep it running even without Blynk connection.

It has been awhile since I worked with this, and I think is still works (although that was with WiFi-as-Shield)… but as you are using ETH-as-Shield, you might have to do some additional searching/experimenting here about any issues or workarounds when using Blynk.config() with an ehternet connection.

the additional code must be placed before the blynk.run?

The key is the Blynk.config() which is non-blocking if the connection is not made, unlike Blynk.begin() Followed by any connection detection/reconnection code options.

As the void loop() needs to be as clear as possible and thus repeatedly runs so fast, the order of code in it is generally irrelevant. But feel free to experiment with placement order.