C#, problem with my reverse string method not working in main method

I haven't examined your string reversing algorithm at all, but in these two lines:

Console.ReadLine();
answer = aString.reverseStr("");

You get a string from the user with ReadLine(), which you simply throw away since you didn't store it in a variable. Then you are asking reverseStr() to reverse a blank string, which is probably not what you want.

It should look more like:

string response = Console.ReadLine();
answer = aString.reverseStr(response);

Or you could combine the two in one line:

answer = aString.reverseStr(Console.ReadLine());

You don't do anything with the user's input:

Console.ReadLine();

And you're reversing an empty string:

answer = aString.reverseStr("");

Store the user's input and reverse that instead:

var input = Console.ReadLine();
answer = aString.reverseStr(input);

you can reverse string in main method like this

 public static void Main(string[] args)
    {
        Console.WriteLine("enter a string");
        string stng =Console.ReadLine();
        char[] revArray = stng.ToCharArray();
        Array.Reverse( revArray );
        Console.WriteLine(revArray);
        Console.ReadLine();
    }

Unfortunately, there are no built in functions to do that. You could use a char array and reverse the array and then get the string back.

using System;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            string input;
            string output;
            MyStr aString = new MyStr();
            Console.WriteLine("enter a string");
           input = Console.ReadLine();
            output = aString.reverseStr(input);
            Console.WriteLine(output);
            Console.ReadLine();
        }
    }

class MyStr
{
    public string reverseStr(string Myname)
    {
        char[] arr = Myname.ToCharArray();
        Array.Reverse(arr);
        return new string(arr);
    }

}
}

Alternatively, instead of MyStr class, you could define an extension for string. And your calling statement will be as simple as

string output = input.Reverse();

Extension method as follows and it can be used everywhere without instantiating a class by just using string variable.

public static class StringUtility
{
    public static string Reverse(this string input)
    {
        char[] arr = input.ToCharArray();
        Array.Reverse(arr);
        return new string(arr);
    }
}

You are feeding the method with an empty string aString.reverseStr(""). You must read the user input into a variable and pass this one to the reverse method.

string input = Console.ReadLine();
answer = aString.reverseStr(input);

You can remove the j variable as you are not using it. The Console.WriteLine("String in reverse: "); should be in the main method, where all the other console input/output is. The reverseStr method should only do the reversing stuff and not have side effects. This would be cleaner.

You can pass a parameter through the constructor like this

class MyStr
{
    private string _name;

    // Constructor. Has same name as class. Doesn't have a return type.
    public MyStr(string name)
    {
        _name = name;
    }

    public string ReverseStr()
    {
        string temp = "";
        int i, j;

        for (j = 0, i = _name.Length - 1; i >= 0; i--, j++)
            temp += _name[i];

        return temp;
    }
}

And you would call it like this

var myStr = new MyStr(input);
string result = myStr.ReverseStr();