Hour from DateTime? in 24 hours format

So i have this DateTime? and what i want to do is to obtain the hour but show it in 24 hours format.
For example:
If the hour is 2:20:23 p.m. i want to convert it to 14:20 and that's it.

I'm working with Visual C#. Any ideas please, thank you.

I have something like this

public static string FormatearHoraA24(DateTime? fechaHora)
{
    if (!fechaHora.HasValue)
        return "";

    string retornar = "";
    //here goes what i need
}