Essentially there are only two valid possibilities after eliminating software as the cause.

1. The GPS antenna assembly is not making contact with the main board properly. Maybe some gunk on the pads, or an alignment problem.

2. The GPS chip is dead.

Number 2 sounds more likely given your description.

The L1 & L2 service manual gives you a good guide on how to actually get at the GPS antenna, L3 & L4 show you what test equipment you can buy from Nokia to figure out how dead the GPS chip is. These manuals can be found on the net in PDF.