Fading LED with button control

Hi guys.

Just got myself a first arduino mega and sorry for the very amateurish question.

Basically, what I want to do is that the LED fades out to totally dark, and when the the button is pressed, the LED restarts fading out.

The circuit is quite easy: pin 12(pwm) -> resister -> LED -> ground; pin 8 -> push button -> pin2.

The problem is that LED fades out again and again, and the button is not working.

Thanks for any help

int pwmout = 12; //Pin 12 as PWM output pin
int out = 8; //Pin 8 as HIGH pin for interrupt
int brightness = 255; //variable for brightness

void setup ()
{
  pinMode(pwmout,OUTPUT);
  pinMode(out,OUTPUT);
  digitalWrite(out,HIGH);
  attachInterrupt(0,reset,RISING);
}

void reset ()
{
  brightness = 255;
}

void loop ()
{
  if (brightness != 0)
{
brightness = brightness-5;
analogWrite(pwmout, brightness);
delay(25);
}
}