I am not familiar with the Piva 350 or any Piva model for that matter, but I do have some suggestions. My old Magellan receiver needed to be reset from time to time. There was a button in the back that required a pen to push down the reset button for about 5 seconds. If you've already tried that, perhaps you could completely run down the battery so that it would re-boot? I also saw somewhere how it takes a memory card - if you have one in, try removing it so see if there is an issue with the card, reader, or improper file that's freezing the system.
Sorry that I wasn't able to dig up some 'official support' since the company seems to be no longer in business, but I hope you get it figured out.