problem reading .txt file in C# starting from (after immediate empty whole line) to (the next empty whole line)

I am trying to randomly read a huge .txt file. It has tons of paragraphs separated by a full line of empty space prior and post each paragraph. I would like each time I randomly read that it pulls up a full intact paragraph without any characters or words missing for the sake of context. I appreciated the help in advance.

I added a for loop just to test it out and see if I can at some point include a way to recognize consecutively running empty space. That would only work post already selected the starting point obviously if applied.

public static string GetRandomLine(string filename)
{
    var lines = File.ReadAllLines(filename);

    var lineNumber = _rand.Next(0, lines.Length);

    string reply = lines[lineNumber] + lines[lineNumber + 30];

    for(int a = 0; a <= 30; a++)
    {
        reply = reply + lines[lineNumber + a];
        //The increment on the lineNumber is because I wanted to receive
        //more than one sentence for context purposes
    }

    return reply ;
}