Had a look through the FAQ,s here http://www.garmin.com/support/faqs/search.jsp but couldnt see much other than mabye updating the software for it. On my brotherin laws Magellan if it got screwed up you were able to reset it by a number of key presses, If you got the manual have a look through it,or you can download it from Garmin
If you have a external antennae i would be checking that first.
Or mabye contact there tech advice here
http://www.garmin.com/contactUs/techSupport.jsp
Cheers twoducks