Declare variables in a public class

Move the close bracket!

public static class Data
  {
      public const int MAX = 5;//Defining the stack value

     public static int top = -1;//Stack is empty
     public int[] stack = new int[MAX];//Declare an array called stack
  }

This is the totality of your Data class, because the close bracket ends the definition.
You need the push, pop and display methods to be within the Data class, so move the close curly bracket below the Main method.

Here is the code,display doesn't work otherwise the push and pop work perfectly fine!! Thanx so much for helping me out :)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace _2
{




class Program
{
public static class Data
{
public const int MAX = 5;//Defining the stack value

public static int top = -1;//Stack is empty
public static int[] stack = new int[MAX];//Declare an array called stack
}

public static void push()
{





int element;


if (Data.top==Data.MAX-1)
{
Console.WriteLine("Stack Overflow");
return;
}

else
{
Console.WriteLine("enter the number to be pushed");
element = int.Parse(Console.ReadLine());
Data.stack[++Data.top] = element;


Console.WriteLine("The number is pushed successfully", element);

}
return;
}

public static void pop()
{

int y;



if (Data.top == -1)
{
Console.WriteLine("Stack underflow");
return;
}

else
{
y = Data.stack[Data.top];
Data.stack[Data.top--] = 0;
Console.WriteLine("The element has been popped", y);
return;
}
}

public static void display()
{



int i;
if (Data.top == -1)
{
Console.WriteLine("Stack is empty");

return;
}
else
{
Console.WriteLine("Elements are :\n");
for (i = 0; i <= Data.top; i++)
{
Console.WriteLine("\n",Data.stack[i]);
}
return;
}
}



static void Main()
{

int ch;
do
{
Console.WriteLine("Enter Your choice \n1.Push\n2.Pop \n3.Display \n4.Exit");
ch = int.Parse(Console.ReadLine());



switch (ch)
{
case 1: push();
break;

case 2: pop();
break;

case 3: display();
break;

case 4: break;

default: Console.WriteLine("Invalid Selection");
break;
}
} while (ch != 4);
}
}
}