ok, prees the top left hand corner of the screen, and at the same time switch it on, keep you finger on the screen,
you should eventually get a box pop up, there will be an option to clear user data, or something like that. select that option and it should reboot, upon startup it will ask for your location and stuff, complete that section and see what happens.
doing this reset will get rid of any corrupt gps data, if this doesnt work it is most likely the antenna at fault as there is a compnent inside that is prone to failure.
hope this helps you