Relactation is re-establishing your milk supply after your baby has been fed formula.
You may want to restart your milk supply days, weeks, months or even years after stopping feeding. If your baby has been on formula for several weeks and your milk supply has dried up, start it up again by putting him to the breast frequently. He may be frustrated at first by feeding at an empty breast, partly because the shape of the nipple isn't such a strong stimulus to sucking as the shape of a rubber teat.
You can try two things. Either let him have some formula from a bottle to satisfy his initial hunger, then let him feed from you. Or give him formula from a cup or spoon, so avoiding the stimulus of the rubber teat, then let him feed from you. Once you start to produce some milk, try to get your let-down working before putting your baby to the breast.
Express Milk Regularly
After each feed, express or pump your breasts to encourage your milk supply to build up. Remember that the more often your baby breastfeeds, the more quickly your milk will reappear. By about two weeks you'll probably be producing enough milk to be able to do away with formula. Remember plenty of skin-to-skin contact and let your baby comfort suck whenever he wants.
The keynote to success is confidence. The experience of women all over the world is that their breasts are capable of producing milk again even after several years of being dry, as long as they have enough stimulation.