Candy isn't an easy to fix thing in most cases you can't just add watery and reboil.How hot did you get it? If the material is soft you should be able to just add some hot water and reboil to the correct temp.
100% correct a candy thermometer is a must have. Infrared thermometer would be the best.i think its pretty important to have a "candy thermometer ". Temp is pretty critical when making hard candy i believe