hi I undercooked my batch of rock candy and was wondering if there was anything I could do about it because I don't want to have to make a new batch. Am I able to put it back in a pot and heated back up because I undercooked it? If not is there anything else I could do to make it harden? Because it will not harden (because it is undercooked). Thanks!

You can certainly try - just heat it on a really low temperature to melt it then bring back to the boil and bring it up to the right temperature, hopefully that will do the trick for you.