After a software update you need to take your satnav outside (with a clear view of the sky in all directions) and give it about 5 minutes to acquire satellites. (It's unlikely to be able to do so indoors, even if you put it next to a window).
If you've tried that, without any success, you probably need to do a master reset. The exact instructions vary between models. Go here:
http://www.garmin.com...support/searchsupport
Put 'acquiring satellites', together with your model number (e.g. 'c300') into the search box. You should find the instructions you need among the links which are produced by your search.
Chris