Grape Juice left out overnight, now has fizz

I'd be very wary of this, if only because it seems to have fermented remarkably quickly. The fizz is likely the result of carbon dioxide being produced by yeast eating the sugars in the grape juice; this is the same process that carbonates beer or sparkling wines.

The thing is, most of these yeasts are introduced deliberately, and they take a while to do their job. In my experience with home brewing, it would generally take at least a few days for the yeast to begin producing significant carbonation. If you truly did just leave this out overnight and it's already noticeably carbonated, that seems to be a very aggressive or efficient strain of yeast. Since it's impossible to know for sure, the safe thing to do would be to throw it out.

Wild fermentation of this kind is used to produce certain beer styles, and other fermented goods such as sauerkraut, so it's not unsafe by definition. I would exercise caution here only because that level of carbonation seems very unusual to me, and grape juice is unlikely to be precious enough to justify the possible hazard.