Help with loops

Question

The problem
Write a console-based application that asks the user to type a
vowel from the keyboard. If the character entered is a vowel,
display "OK"; if it is not a vowel, display an error message.
Be sure to allow both uppercase and lowercase vowels. Th e
program continues until the user types '!'.

I been doing it for last 30 minutes, and I just cant figure out how to do it....

This is as far as I got

            int count = 0;
            double userInput;
            string userInputString;

            Console.WriteLine("Enter vowel");
            userInputString = Console.ReadLine();
            userInput = Convert.ToChar(userInputString);

Help?