Atmega 2560 does not start after blackout

Hey there I have a big problem concerning my ATmega2560!

It use is to controll a big Display for a gym hall.
Therefore its communicates with an raspberry Pi via Uart.
All works fine but after a blackout the ATmega2560 wont start again.
You have to remove the wires from the Uart, so that the ATmega 2560 can start proberly again.

Specs of the Programm: I recive the the Data from the raspberry pi via the rx interrupt routine.
I also use Compermatch Interrupts of other timers.

My question is: Does a blackout work like an reset?
Could it be that the Interrupt routine of the Uart causes the fail start?

NoName:
You have to remove the wires from the Uart, so that the ATmega 2560 can start proberly again.

Really, you have to do all that ?

What happens after the 'blackout' if you press the reset button in the ATmega 2560 ?