Hypoglycemia is low blood sugar that affects the central nervous system.
It is mostly a concern for the small breeds.

Stress is the leading cause.
It can cause death so do not take this lightly.

Another major cause is not feeding your puppy often enough.
This is why I tell you to feed your puppy 4 times a day but I highly recommend that you 
leave the food out all day and let your puppy eat when he is hungry.  That way you are leaving nothing to chance.

                                                                                                       How to Tell if your Puppy has Hypoglycemia

If your puppy is getting low blood sugar, you will first notice that he is listless and lethargic.  Get something sugary on his gums and immediately call the vet.

These are the best choices in order: honey, pure maple syrup, or sugar. 
There is usually no warning and missing a meal or not having water can trigger it.
Also, it is important to reduce the stress for your puppy as much as possible.

The symptoms will continue on with muscle weakness, vomiting, diarrhea, tremors, seizures coma and then death.
It can be quick.

As a side note, I have never had this happen!