unity - Destroy chunked block like Minecraft

I am trying to do a Minecraft-like game (like every one have already tried I guess) and i went across the problem i expected: lag with world Generation. I did lot of searching but i can

I tried to combine them into a bigger mesh, which work, but not i can destroy a single block. So how can I place/destroy chunked (mesh combined) block? If i cant, can someone clearly explain me a solution (i am not fluent in english nor with programming expressions, i can code tho) and how to do it (with a script example if possible :) ) ? Thanks in advance !

I generated my world using 2D perlin noise. Once i have the blocks, i split them into 16x16x100 chunk meshes.

Before Making chunks, i first tried to combine all of them into one block (using the unity manual), but the result is only one cube placed at 0,0,0. Where are all the other cubes ?

using System.Collections;

using System.Collections.Generic;
using System.Linq;
using UnityEngine;
public class ThisWorldGen : MonoBehaviour {

public GameObject blockPrefab;
public GameObject grass;
public GameObject dirt;
public GameObject rock;
private string[] blockTag; 

private GameObject stockin;


public int amp;
public int freq;
private Vector3 mypos;
[SerializeField]
private Material material;
private Vector3 blockLocations;


public List meshes = new List();

void Start()
{
    Generate();
}

void Generate()
{
    amp = Random.Range(0,100);

    freq = Random.Range(30,100);
    mypos = this.transform.position;
    int cols = 100;
    int rows = 100;
    float startTime = Time.realtimeSinceStartup;    
    #region Create Mesh Data
    MeshFilter blockMeshes = Instantiate(blockPrefab, Vector3.zero, Quaternion.identity).GetComponent();

    for(int x = 0; x < cols;x++)
    {

        for(int z = 0; z < rows;z++)
        { 
            float y = Mathf.PerlinNoise ((mypos.x + x) / freq,(mypos.z + z)/freq) * amp;
            y = Mathf.Floor(y);
            for(float hy = y; y > 0.0F; y -= 1.0F)
            {           
                blockMeshes.transform.position = new Vector3(mypos.x + x,y,mypos.z +z);//move the unit cube to the intended position
                meshes.Add(blockMeshes);        
            }
        }

    }
    int i = 0;
    MeshFilter[] listMeshes = new MeshFilter[meshes.Count];
    foreach(MeshFilter cubesToAdd in meshes)
    {                   
        listMeshes[i] = cubesToAdd;
        i++;
    }
    CombineInstance[] combine = new CombineInstance[listMeshes.Length];
    int w = 0;

    while(w < listMeshes.Length)
    {   
        combine[w].mesh = listMeshes[w].sharedMesh;
        combine[w].transform = listMeshes[w].transform.localToWorldMatrix;
        listMeshes[w].gameObject.SetActive(false);
        w++;
    }
    Debug.Log (combine.Length);
    transform.GetComponent().mesh = new Mesh();
    transform.GetComponent().mesh.CombineMeshes(combine);

    transform.gameObject.SetActive(true);
    #endregion
    Debug.Log("Loaded in " + (Time.realtimeSinceStartup - startTime) + " Seconds.");
}
}

(the world was generating randomly and perfectly before trying to merge the meshes)