I learned from my lactation consultant that when a baby *suddenly* stops nursing, it's not self-weaning, they call that a "nursing strike" - (you know, like workers being on strike.) It's very rare for a baby under 18-24 months to wean all by themselves, and it happens gradually.
The best way to handle it is to get them back to nursing, because it's still the best food for them. (Unless you absolutely HATE nursing and you've been dying to get to this point.) But most moms that have made it as far as you have (congratulations, by the way), want to make it at least up to 12 or 18 months.
Either way, you'll want to PUMP to stay comfortable (not engorged), and reduce the risk of a plugged duct, mastitis, and (unless you want to quit) keep up your supply. To maintain the supply, pump more frequently - get an inexpensive manual pump if you don't already have one. Isis makes one under $50.
If you want to end nursing now, pump just enough to relieve the pain and pressure. If you don't own a pump, hand expressing should be enough to get over the pain, (Yeah, I mean like milking a cow. It will probably take a little getting used to - you may need to get in the shower and try it under the warm water, but once the pressure is off, you'll feel much better.)
To get her back on, try nursing as she's dropping off to sleep or right when she wakes up. There is more info here about nursing strikes and tips to help coax a babe back to the breast:
http://www.kellymom.com/bf/concerns/baby/back-to-breast.h...
Make sure you check out the links at the very bottom of the page specifically about nursing strikes, including "Surviving a Nursing Strike."
Good luck!!!!!