On a standard Arduino pin 0 will not work as an interrupt, only pin 2 and 3. So your interrupt is never activated, never setting resetP to 1, thus never satisfying the if to call your subroutine.
Interrupt 0 is on pin 2. attachInterrupt() uses the interrupt number, not the pin number.
The code uses Serial3 so it's running on an Arduino Mega of some kind. That means interrupts 0-5 are available from pins 2, 3, 21, 20, 19, and 18 respectively.