Vitamin C is a vital nutrient for your adrenal glands, and if you get low your ability to deal with stress can quickly diminish.
It's also a water soluble nutrient so isn't stored in the body which means you can get depleted pretty quickly and is why you need to eat fruit and vegetables daily.
Vitamin C is also very important for the health of endothelial cells such as those found in the lining of the mouth so early indicators of low vitamin C status are mouth ulcers, cold sores or cracked sore lips.
If you get any of these it's time to take action - for me this means drinking some fresh fruit and vegetable juices, I'll also add half a squeezed lime to every hot drink I have.
Remember also that sugar inhibits vitamin C absorption so cut out sugar til you've recovered.