Reheating the sauce containing the meatballs, the meatballs alone or the sauce alone would not be a good idea. 18 hours is a long time for bacteria development. I don't think you would necessarily get sick from reheating it and eating it but because of the time it stayed outside safe temperature and the bacteria development that followed, I cannot recommend such course of action. My recommendation is to throw it all away and be more careful next time.
When you cook, you should ideally cover the food partially and put it in the fridge right away if you own a large commercial fridge, and 20 minutes later if you own a normal household refrigerator. If you cover the food/sauces completely, chances are it will turn sour and goes bad. Partially covering hot preparation is paramount when cooling it down.
Hope I answered your question.