Al-jawab billahi at-taufeeq (the answer with Allah's guidance)
Food and drink left over night should be covered.
Narrated Jabir (RadhiyAllahu Anhu):
Allah's Apostle said, "Extinguish the lamps when you go to bed; close your doors; tie the mouths of your water skins, and cover the food and drinks." I think he added, ". . . even with a stick you place across the container." (Bukhari, Tirmidhi)
The command of this Hadith is based on the 'Compassion' of Prophet (Sallallahu Alaihi WaSallam) and not a 'Necessity'. The Hikma of the Hadith is that so nothing harmful or filth enters the food or drink.
If it is left uncovered, then there should be no problem to consume it if there is no traces of harm or filth (i.e. smell, colour and taste).