A simple speculation here:
Vampires can go into a deep sleep for a long period of time. This is fact. I am inclined to believe that they must forgo drinking blood for awhile to go through this process. Again, it's not concrete fact but by seeing different scenarios in lore, we can only assume. We know that vampires can sleep off for centuries or even thousands of years like our dear Serana.
QUOTE
Vampires were believed to be extinct in Morrowind for centuries. Dunmer culture has a special hatred for vampires, and in earlier times the Ordinators and Buoyant Armigers hunted them to extinction. In recent years, however, vampires have either begun to sneak into Morrowind, or long-dormant ones have been awakened.
-Legions of the Dead.
It is possible that Rona Hassildor, rejecting the blood potions from Mina(The Pale Lady) was getting for her, willingly slipped into a coma so she wouldn't have to live as a vampire. Hence the
QUOTE
What? Janus... please, no. Let me sleep. Please, let me sleep.
The only way to bring back one of these long-dormant vampires is by magic or maybe even by blood(ever wondered why your characters gets his/her hand stabbed through with that spike right before freeing Serana? Your character's blood must have been the first blood she's tasted in over a millenia, which explains one of the many reasons she might be so attached.

RP-wise, my kids Draken and Raven willingly go comatose when stuff has hit the fan. Badly. Only under urgent irreversable circumstances do they leave the world and return centuries later to try again, though they aren't always pleased how the world has changed since then. Ehem, Thalmor supremacy and Empire's fall anyone?
This post has been edited by Darkness Eternal: Feb 23 2013, 02:29 PM
And yet I am, and live—like vapours tossed.
I long for scenes where man hath never trod
A place where woman never smiled or wept
There to abide with my Creator, God,
And sleep as I in childhood sweetly slept,
Untroubling and untroubled where I lie
The grass below—above the vaulted sky.”