My partner made Elderflower cordial with elderflower, sugar, lemon, a very small amount of cider vinegar and water about 3 months back. Last night I drank about 3/4 pint of it instead of booze as I am trying to go dry for September. This morning I feel as though I have a hangover? Can the sugar have turned to alcohol? It was ver sweet. Help?
As far as I was able to find, nothing in elderflower would really stop yeast, and acidic nature might hamper them a bit, but is not enough to stop them entirely. So yes, a bit of fermentation might happen.
A short google search reveals: How to make Elderflower Wine
There really isn't that much difference between what you did and the recipe described in the article. Yes; if it has sugar, yeast can turn it into ethanol.
Also, as you created less-than-ideal fermentation conditions (lack of proper nutrients, not enough o2/ppm, unknown cell count), the production of methanol, fusels and all other less-than-ideal alcohols would actually increase - so, you may have inadvertently created hangover fuel.
On the other hand, the vinegar may contain acetobacter, so it might eventually turn into vinegar.