I don't think i'm getting you. So a whole 'clump' of the plant broke off...roots and all? If that is the case, it'd be similar to 'dividing perennials.' Just repot the section that came off in another pot for a separate plant or plant both clumps in a bigger pot with some space between the two clumps for a bigger plant. Nurseries usually put more than one bulb (more correctly a 'corm')when they're forcing blooms for sale so the pot looks more full. Chances are, the pot had more than one corm. If the callas are coloured (not white), then they won't stand being constantly wet - let the pot dry out a bit (top inch). However, if the callas are white, you can keep the pot a bit more moist.
Eventually the
flowers will fade. Continue to water regularly with balanced fertilizer (if you have some, if not that's ok). In a few weeks or so, the leaves will start yellowing - don't panic, it's supposed to. Once all the leaves have died, carefully dig up the corm, and store it dry for three months or until a bud starts to poke out. Then replant. Water thoroughly once, and only water when the pot has dried or when a shoot pokes above, whichever comes first.
Most people don't know callas (and gloxinias, cyclamens, etc.) can be saved so they just throw out the pot thinking they killed the plant...and they go buy a new one. It's natural for the plants to go dormant. The corms will get bigger after each season, and you will eventually be able to break each corm into smaller ones.