[SOLVED]Keypad -controlled LED program not giving Serial Window output

I've gone and posted this in the troubleshooting section like the bonehead that I am. Please don't report me. It was a real brain fade.
So, I just got myself a 4x4 keypad, and I tried it by running a sample program to print whatever character I press on the keypad, onto the Serial Window. It worked fine.
Then, just for the heck of it, I tried to run another program, which would do the following:
If I press 'A' on the keypad, it would light up the red LED.
If I press 'B' on the keypad, it would light up the yellow LED.
If I press 'C' on the keypad, it would light up the green LED.
If I press 'D' on the keypad, it would light up all of the LEDs.
If I press '*' on the keypad, it would light the nth LED n times.
Anything else would just loop the whole thing back.
This is the code I wrote:

#include<Keypad.h>
int ledpin[]={10,11,12};
const byte rows=4; 
const byte cols=4; 
char Keys[rows][cols]={
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};
byte Row[rows]={9, 8, 7, 6}; 
byte Col[cols]={5, 4, 3, 2}; 
Keypad keys = Keypad( makeKeymap(Keys), Row, Col, rows, cols); 
void setup(){
  Serial.begin(9600);
  for(int i=0;i<3;i++){
    pinMode(ledpin[i],OUTPUT);
  }
} 
void loop(){
  Serial.println("Enter any character from keypad");
  while(Serial.available()==0){
  }
  char K = keys.getKey();
  if(K){ 
    switch(K){
      case 'A':
        digitalWrite(ledpin[0],HIGH);
        delay(200);
        digitalWrite(ledpin[0],LOW);  
      break;

      case 'B':
        digitalWrite(ledpin[1],HIGH);
        delay(200);
        digitalWrite(ledpin[1],LOW);  
      break;
    
      case 'C':
        digitalWrite(ledpin[2],HIGH);
        delay(200);
        digitalWrite(ledpin[2],LOW);  
      break;

      case 'D':
        for(int j=0;j<2;j++){
          digitalWrite(ledpin[j],HIGH);
          delay(200);
          digitalWrite(ledpin[j],LOW);  
        }
      break;

      case '*':
        for(int k=0;k<3;k++){
          for(int l=0;l<=k;l++){
            digitalWrite(ledpin[l],HIGH);
            delay(200);
            digitalWrite(ledpin[l],LOW);  
          }
        }
  
      default:
      Serial.println("Let's try this again...");
      break; 
    }
  }
}

The only output on the Serial Window I'm getting is "Enter any character from keypad". I think it's something wrong with the code because the connections seem straightforward enough to me.
I'm connecting the Led pins Red, Yellow, Green to pins 10,11 and 12 on the Arduino, and pins 2-9 are reserved for the keypad. I didn't change anything from the keypad circuit earlier; I only added the three LEDs to the circuit. It's really bugging me for the last half an hour or so. Thanks in advance, all.
EDIT:
Thanks a ton, guys. The program is finally working. Your responses have been really helpful. Turns out, you were all right: I hadn't written the line to print anything on the serial window. Here's the updated code, if anyone is interested.

#include<Keypad.h>
int ledpin[3]={10,11,12};
const byte rows=4; 
const byte cols=4; 
char Keys[rows][cols]={
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};
byte Row[rows]={9, 8, 7, 6}; 
byte Col[cols]={5, 4, 3, 2}; 
Keypad keys = Keypad( makeKeymap(Keys), Row, Col, rows, cols); 
void setup(){
  Serial.begin(9600);
  for(int i=0;i<3;i++){
    pinMode(ledpin[i],OUTPUT);
  }
} 
void loop(){
  char K = keys.getKey();
  if(Serial.println(K)){ 
    switch(K){
      case '0':
        digitalWrite(ledpin[0],HIGH);
        delay(200);
        digitalWrite(ledpin[0],LOW);  
      break;

      case '1':
        digitalWrite(ledpin[1],HIGH);
        delay(200);
        digitalWrite(ledpin[1],LOW);  
      break;
    
      case '2':
        digitalWrite(ledpin[2],HIGH);
        delay(200);
        digitalWrite(ledpin[2],LOW);  
      break;

      case '3':
        for(int j=0;j<3;j++){
          digitalWrite(ledpin[j],HIGH);
          delay(200);
          digitalWrite(ledpin[j],LOW); 
        }
      break;

      case '4':
        for(int k=0;k<3;k++){
          for(int l=0;l<=k;l++){
            digitalWrite(ledpin[k],HIGH);
            delay(200);
            digitalWrite(ledpin[k],LOW); 
            delay(200); 
          }
        }
    }
  }
}

Except for the default case, there are no other serial prints but "Enter any character from keypad" in loop().

Thank you for the reply, groundfungus. I should have been clearer. What I meant was that my Serial Window refuses to accept any input from the keypad. When I press 'A', it doesn't show up on the screen. What do I have to change in the code, to get it to work?

As groundfungus implied, you don't have a Serial.print statement to print the character that was pressed on the keypad.

Insert a serial print after acquiring the key.

char K = keys.getKey();
Serial.println(K);
  if(K){

When I press 'A', it doesn't show up on the screen.

Where are you expecting to see it ? On the input line or in the output window ?

Insert a serial print after acquiring the key.

Not there, though. Move it inside the if(K) block.

Not there, though. Move it inside the if(K) block.

Right, otherwise Serial gets flooded (palm to forehead).

Oh, crapbaskets. I forgot that part. I thought that the whole getKey() function took care of that. Thanks for the reply, guys.