What model GPS device?
For most of them, I think it only shows the basemap when zoomed right out, or looking at an area not covered by any other maps. Usually the basemap is not very detailed or accurate, ie it only has a few main roads and cities. It shouldn't affect what roads are shown when you are zoomed in.
The basemap may also contain elevation data, which it uses for showing hill shading or elevation profiles.
If you run the firmware updater software, it should reload the missing basemap.