Hmmm... I've never tried it, but one of my (many!) breastfeeding books says you CAN return to breastfeeding, though it seems to take a lot of effort and commitment, depending on how much time has gone by since you stopped. Do you have a local La Leche League group? Or check your local library's parenting section for books on breastfeeding for help, too.
Apparently, putting your baby to the breast frequently will bring back your milk, but to keep the baby from getting frustrated, you can set up a thin tube from a bottle of formula for the baby to take along with it. Or, you can use a breast pump. I don't know anyone who's done this, and never tried it myself, but it doesn't seem to hurt to try it
Or, check out the following website for information -- inducing lactation is halfway down the page:
http://www.sahmiam.org/adopt.html
Good luck!!