The best way to reheat frozen spaghetti is to defrost it in the fridge overnight, then add 2 teaspoons of water to it (per cup of spaghetti) and place the noodles in a microwave-safe dish. Microwave on full power for 30-45 seconds, then remove, stir, and return to microwave at 30-second intervals, as many times as needed, until it's heated through. Alternatively, once your noodles are defrosted, you can put them in a colander/strainer and dip them into boiling water for approximately 30 seconds or until they are warmed up.