Any time you want to use millis(), delay(), or Serial.xxx() inside an interrupt, you have call sei(); first.
This should work, but it might freeze the arduino if the interrupt is called repeatedly because it would keep getting called in the middle of the delay and would never return from the function. You could lock it like so:
volatile char isRunning;
void myInterruptFunction(void)
{
if (!isRunning) {
isRunning = 1;
sei();
delay(debounceDelay);
//make state changes
isRunning = 0;
}
}