Hi,
Yesterday I had a headache 'cause of my teeth problems and I was expecting to go to the gym and burn 1200 calories, so what happened is that I didn't go and those 1200 that I over-ate probably transformed into fat.

But I would like to know if that energy that has been transformed into fat is harder to burn NOW, than yesterday?

Missing a day's workout isn't going to make as huge as a difference as you may be thinking. Fat is fat is fat...and it all burns off the same way-proper diet, and regular exercise. Missing a day's workout isn't going to make a difference in how many calories you burn today during your workout really.

You must also understand that the body burns calories constantly-by eating, sleeping, digesting food, breathing-every movement you do through the day burns calories. Exercise burns "extra" ones-or more per minute than you would just doing regular activity.
You burn calories during a workout-but that is not the only time you burn them.
Also-just by missing a single workout-it doesn't mean that everything you ate that day is going to suddenly tranform to fat. That isn't how it works-it's more complicated than that. The key is to eat better continually over the long term, and get exercise in regularly for results.