Graham - what your describing sounds like you somehow managed to erase the street level maps and POIs in your Nuvi.
If you connect the GPS to the computer with your USB cable, you should at the contents of the Nuvi directories and find a file called GMAPPROM.IMG or GMAPSUPP.IMG. These are the street level map files. Generally, these are found in the root directory of the Nuvi - or if yours has / and you use a memory card slot for maps - find it in the /GARMIN directory. GMAPPROM.IMG is the name of the first map set and GMAPSUPP.IMG is the name of the supplementary or second map set (if you purchased & installed one). You will likely have only one of these files - and may be named either. The size of this file depends on the area and age of the map. Current maps of the entire US run over 1 GB in size. If you do not have any maps named GMAPPROM or GMAPSUPP, you will not see any street level maps.
Something happened to delete the maps. Garmin will probably NOT provide free replacement maps for you, unless under warranty. If the devices is trashing maps and other files, you probably want to replace the unit under warranty or not. Garmin will, upon return of your old Nuvi - replace it with a factory refurbished unit with the latest maps installed, for $79. You can take advantage of this service by following this link to Garmin Service
I hope this helps and good luck! Please rate my reply. Thank you.