Hi there... 5 stones is a lot of weight to lose in 3 weeks and it's really not possible to lose that much (5 stones = 56 pounds), however, since you want to lose weight, this would be a good time to get started on a good diet and exercise plan so you can reach the weight you want to be.
I can't help wonder if you misunderstood your doctor about exercising... almost everyone can do "some" type of exercise, even if it's only light walking for 10 minutes/day, yoga (there's even yoga that can be done while sitting in a chair), etc...Perhaps you should check with him again to verify if there are some types of light exercise that would be suitable. Some exercise is necessary for muscle maintenance, otherwise, our muscles will simply waste away.
That said, diet is, often, the biggest portion of weight loss, anyway. The main thing you have to do is figure out how many calories you need/day, based on your current age, weigh and height, then not go over that. Secondly, you have to make sure you're eating healthy foods, not junk food, like chips, candy, cookies and other processed foods, like prepared, boxed foods.
Make sure your meals are centered around fresh veggies, lean protein, very little fruit (to avoid the sugar), healthful fats, etc.
Make sure you eliminate sodas and fruit juice and make water or unsweetened tea your drinks of choice.
If you provide the necessary information, we can help you do the calorie calculations, etc. For best results, you should keep track of everything you eat/drink, in order to make sure you don't go over your calorie budget. MH has an excellent diet diary to help you do this.