If you’ve left out a non-vegetarian dish such as meatballs overnight in the microwave, do not consume it. Two hours is usually the limit for unrefrigerated perishable food. Here, because the meatballs were heated and then left in the warm microwave environment, they probably became dangerously compromised by bacteria within an hour.
When you’re trying to decide if something is safe to eat, remember the rule ‘time and temperature equals bacteria growth’. Usually, you have two hours between when the food is oven-hot or refrigerator-cold, before the bacteria can multiply to the point of making you sick. In the summer, when the temperature is higher, or in a warm place such as a microwave that was hot and then cooled off, the bacteria can grow even faster. So in summer, reduce the two-hour rule to one hour.