Im also confused by this, as diet plans such as Cambridge or Lighter Life are very low calorie diets and people lose tonnes of weight but then others say people wont lose if they dont eat. Very confused!
Because such rapid weight loss comes at the expense of muscle mass and internal organs rather than body fat, there is a distinct biochemical speed limit for how fast your body can physically move the fat out of your fat cells and both Cambridge and Lighter life attempt to violate this speed limit and end up sacrificing muscle mass on the altar of a lower scale weight.
Muscle mass is active tissue and is what burns calories, so after a starvation diet like Cambridge you're burning less calories, you're hungry as hell because you've been burning muscle to keep alive, and your body is primed to store fat.
These starvation "cures" only last a few weeks because the metabolic damage you sustain tends to kill you if prolonged. Even in the case where you survive and don't develop an eating disorder, your new starvation-induced snail's pace metabolism will be able to maintain obesity at almost unfathomably low calories.
You need to cut calories to lose weight, but the trick is finding the right balance. Go too low, and your body will respond by slowing down your metabolism, slowing the rate at which you lose (note: you will still lose - but there are a lot of reasons not to do this since your body needs fuel and nutrients to function optimally). If you're over-restricting, upping your calorie intake to a healthier level of restriction can help kickstart weight loss a little bit. Also, eating a little more helps fuel you to exercise more.
Those "diet plans" are garbage. They are designed for those who are only looking for the quick fix. Chances are, those people who lose tons of weight will regain it and end up in a yo-yo cycle.
Eat enough, workout, and be patient.
The point is you're trying to lose weight now all over again. Obviously losing so much so quickly wasn't sustainable.
If you are a teen, you should eat no less than 1500 -- if you are overweight and sedentary. If you are overweight and active, you should eat no less than 1800. If you are not overweight, you should not be restricting calories -- exercise (strength training in particular), is a much better way to go to "re-shape" your body.