Are minor roads showing on the screen or just the major ones? Sounds possible that you have no detailed mapping and are running on the basemap. If your detailed mapping (i.e. City Navigator Europe) is on an SD card ensure that it's properly inserted. Failing that go into the settings menu (not sure of the correct path on a 550, sorry) and find 'map info' or similar. This will show what mapping is installed and if it's active or not (check-box alongside; ticked = active).
If there is no detailed mapping listed either your SD card has gone bad, if that's where the mapping is. Or there's been an issue with an update and the mapping file is incomplete or corrupted.
Let us know what you find and we should, hopefully, be able to advise as to what to do.
EDIT: Good point Katnip, had forgotten about the 550 battery in/out fix.