using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var numbers = new [] {

            1,3,4,5,6,4,5,6,8   

        };

        var level = 5;

        var highest = numbers

            .Select((x,i) => new { i, matches = x == level })

            .Where(x => x.matches)

            .Max(x => (int?)x.i);

        if (highest.HasValue) {                                 

            var index = Math.Min(highest.Value + 1, numbers.Count() - 1);

            var next = numbers[index];

            Console.WriteLine($"index : {highest}, next {next}");               

        } else {

            Console.WriteLine("nada");  

        }                       

    }

}