I can understand your frustation. It sounds as if you did a great job and the lesson went well. A positive way to look at this is that you can keep your lesson plan for the next lesson.
This could just be a glitch from the beginning of the year. People may be busy the first week of January and you may see that they turn up regularly as of the next lesson. I think you're anticipating a problem that isn't a problem yet. A good way to approach this is to include lots of pair work in your lessons; that way no matter how many students turn up, you can always do the activities. If only one person shows up, you can take one of the parts or if an odd number of students are present, find a role that someone can take in a group of 3. Don't plan intricate lessons that require swapping partners, etc.
As for the levels, wait and see the others in the class next week. If the other students are beginners, you should talk to someone about moving the intermediate student. If there is no way to move the student, you'll have to plan for mixed ability classes; there are many posts on that topic in these forums.
All the best, Lucy