I get this question a lot. Slowed down your metabolism? – yes. Ruined it? – no, absolutely not. One of the many magnificent things about your body is it's ability to bounce back from things like this.
Dieting lowers metabolism by making your body fear starvation, which could ultimately lead to death. (Remember that your body's #1 job in any given moment is to do whatever it takes to keep you alive.) So when you restrict your calories, your body responds by burning fewer calories, sort of like how the thermostat in your house makes adjustments to keep the temperature stable. Your body is making adjustments to keep your weight stable.
Side note: the combination of restricting calories and increasing exercise is a recipe for shutting down your metabolism, completely shooting yourself in the foot if weight loss was your goal.
Once your metabolism has slowed is it doomed forever? Nope. The cool thing about metabolism is that it almost immediately responds to adjustments in your intake. By increasing your eating to meet your body's energy needs you increase your metabolism. Doing this and getting in physical activity is the most effective way to get your metabolism high and keep it there.
Products (pills, supplements, drinks, superfoods, etc) that advertise to be metabolism boosters are bogus. Don't waste your money, or jeopardize your health, on these products.