Left Hot fudge on counter overnight
This is impossible to say if it is good or not for sure. There are many many factors that come into this, which make it impossible for us to tell - How long it was left out? What you did with it before (did you pour out or dip a spoon in, leave the lid off? etc.). How long has it been stored since you opened it? These are just a few of the more obvious questions... Note that I'm not expecting you to answer these, as answering them still won't allow us to give you a definitive answer.

However for informational purposes:

There are some factors for food safety that play a role here - namely the water activity of the food stuff. This plays a role in how likely bacteria and other contaminants are to grow in the food. Basically the lower the water activity, the less likely they are to grow, and this is due to the availability of water in the food for the contaminant to grow. 

Most bacteria need a water activity around 0.9, but can go as low as 0.7 (1.0 is pure water, pure sugar would be 0.0), yeasts and fungi can grow as low as 0.6, depending on the substrate in which they are growing. 

Based on this document (PDF file), the water activity of your sauce is likely to be about 0.8 (basing this on chocolate syrup in the table), which means that most bacteria will be inhibited, but many yeasts and molds will not be so inhibited.

Yeasts will happily (and rapidly) grow at room temperature, while molds are generally slower growing and less obvious until they reach the fruiting/sporulating stage. In the mycelial phase molds will penetrate large areas of the food before showing the obvious sporophytes (the green/blue spots you might find on spoiled fruit or bread). Picking the sporophytes off does not remove the mold, which will have extended several inches beyond the visible spots.

While yeasts usually only make food less palatable and odd-looking, they are opportunistic pathogens (i.e. if you are sick they are more likely to colonize and make you sicker). Molds on the other hand can produce mycotoxins that can make you very very sick, and even kill if you consume enough.

So - it is possible it is fine, but we can't say for sure. The general rule is: "if in doubt, throw it out".

Edited to add a bit more on yeasts and molds.