You cannot and should not eat ranch that was left out overnight.
Ranch dressing starts to grow bacteria and spoil after 2 hours of being left out at room temperature.
In temperatures above 90 F the ranch dressing can grow bacteria in as little as 1 hour.
Leaving ranch dressing out longer than a few hours increases the risk of bacterial growth which can lead to foodborne illnesses.
If not eating the ranch dressing you should refrigerate it within 1 to 2 hours.
Always keep ranch dressing refrigerated when not consuming or using it.
When Ranch dressing is stored in the refrigerator the ranch dressing typically lasts 1 to 2 months after opening.
To tell if the ranch dressing has gone bad take a sniff of it and it will smell like spoiled milk when it has gone bad.
If the ranch dressing smells of spoiled milk or rancid then it has gone bad and should be thrown out.
If you buy ranch dressing, blue cheese dressing or other salad dressing, or salsa off the shelf, it's fine in your pantry until you open it (as long as it's before the “Use By” date).
However once you open the ranch dressing, blue cheese dressing or salsa it must be refrigerated to prevent spoilage.