Like oatmeal that's hardened in the pan? Usable but resistant to the spoon. Cold. Lumpy.
The characters are like people you used to know. The girl who was your best friend in 6th grade, the guy you had a crush on for a couple of months when you were 20. Your Latin teacher who died from a brain tumor 3 years after graduation. You have the basic facts (glasses, hair color, a certain way of dipping the head, a brand of corrective shoes), but the juice of them--the personal tastiness--is absent, has dripped away while you had writers' block or were involved in your day job or whatever kept you too busy for the novel.
You have to get to know them again, or rather, you have to court them, seduce them, not to your bed but to your virtual pen.
Dear Main Character,
I know it's been a long time, but I've been thinking about you. Can you forgive me for my neglect? I hope we can start again.
yr unfaithful author