have i fried my arduinos i2c connection?

Im connecting raspberry pi to arduino through i2c. It worked fine but suddenly it stopped working and I am afraid something have happened to the arduino.

I have connected:

(Pi) SDA -> SDA (Arduino)
(Pi) SCL -> SCL (Arduino)
(Pi) Ground -> Ground (Arduino)

But when I am running

sudo i2cdetect -y 1

suddenly it returns blank:

   0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --

When I am connecting another sda device to the pi it however gets detected so there is nothing wrong with the pi.

Is there a possibility I can have fried the arduino?

It however gets detected and I if I upload a simple sketch to it for blinking it works with no problem. It is just the i2c who doesnt get detected anymore

I have also written a simple sketch to display the input value from 20 and 21 on my mega (SDA and SCL).

void setup(){
Serial.begin(9600);
pinMode(,INPUT);
digitalWrite(20,HIGH); // turn on internal pullup resistor
pinMode(21,INPUT);
digitalWrite(21,HIGH); // turn on internal pullup resistor
Serial.println("Booted");
}

void loop(){
static bool pin20=LOW;
static bool pin21=LOW;

if(pin20!=digitalRead(20)){
  pin20 = !pin20;
  Serial.print("20=");
  Serial.println(pin20);
  }
if(pin21!=digitalRead(21)){
  pin21 = !pin21;
  Serial.print("21=");
  Serial.println(pin21);
  }
}

this returns 0 when when I connect pin to the ground and 1 if not so the input does not seem to be fried or can it be that anyway? Any other way to test arduinos sda and scl?

It seems more likely that the 5v Mega may have damaged the 3.3v RPi - but I have no experience of RPis nor of interfacing I2C between different voltage microprocessors.

...R