Depends where the data was stored originally. Some folk store the maps and waypoints on the unit memory, some folk put it on the SD card. If yours was on the card and that card is now gone .... then so is the data.
Personally, I keep maps and map data on the unit memory and MP3's on the card.
Had a simular problem, rang garmin uk customer services today who fixed it over phone, a little complex and took 20mins. Base mapping had somehow been deleted . Very helpful so suggest you do same and have your garmin, pc and connection lead all to hand. http://www.garmin.com/garmin/cms/site/uk/contact
sent my zumo to garmin
week later one new zumo in the post
well done garmin
good helpline
excellent customer service
some companys could learn by there example