Hi Raghavendra,
Sorrry for the delayed response. The trouble starts when you press a key right? There is no check for key debounce in that code. If you use keys with long bouncing time this code may crash. From your post I make out that could be the probelm, to confirm modify the code slightly as shown below and try again.
Inside the while (1) loop of the main() function
while(1)
{
row1=row2=row3=row4=0;
if(col1==0)
{ //*****add these lines**********//
delay_ms(30) //add a 30 millisecond delay and check the key again
if (col1==0)
keypad();
else;
}