Help with garmin Nuvi 770

[luke scott]

i have a nuvi 770 thats used in my wifes car. Recently had maps updated by a guy here in Dublin and he put everything on a card. All worked well but then the other day she gets a message on screen ....."Unable to calculate route. No roads near starting point". It's calculating up to 27% of the route then saying unable to calculate. Although it just calculated a shorter jouney of around three miles locally ? ? ?
WIll I do a reset on it ?
If I do will I loose her saved favourites ?
Does it sound like all the maps on the card are gone ?
How do I do a reset on it ? Press & hold screen ? Where & how long ?

Thanks.