Bob:  You are new to bees, and unfortunately "it shows".  This is almost
April, and apparently you did not examine INSIDE YOUR HIVE from about October.
Only God can diagnose bee problems from outside a colony!  I demand that all
my "students" examine the inside of a colony at least once/month in Nov.,
Dec., & Jan. and at least twice each month in Feb. and March.  Remember that
next winter!
I can't, and I doubt if anyone can describe what happened to your bees; but I
have two STRONG suggestions: Your symptoms are classic symptoms of loss by
tracheal mite.  80-90% of tracheal mite death occurs in Jan. and Feb. leaving
practically no dead bees in the hive and lots of honey.  I noted that you did
not mention treating with Menthol, which is the only approved killer of
tracheal mites.  Grease patties do not KILL, only control tracheal mites, and
they must be used 12 months of the year (even during the nectar flows), and
the grease patties must be free of Terramycin.  My second choice for the
demise of your colony would be death of the queen.  Queens have suffered much
change over the past 20 years in poor breeding because of drone loss resulting
from mite kill of feral bees, and the stresses that were not present years
ago; e.g. parasitic mite syndrome, virus, medications, increased nosema or
chalk brood, and poor handling by beekeepers.  You can't keep bees like Daddy
did!  Times have changed.  You have a computer and e-mail - Daddy didn't!
Daddy's bees did not have mites, nor exposed to Apistan, menthol, FGMO,
essential oils, and you name it.
What you need is a lot of Reading and good advice.  If you want, e-mail back
your address (just the town is enough) and I will see if one of the New Jersey
Master Beekeepers is close by who can help you FREE of charge of course.
Don't get discouraged, just learn more.   LEARN LEARN LEARN.
George Imirie, 65 years with bees in Maryland