Probably the easiest way to go about it, is to pick up a new compass, for less than $20, but don't install it before you
remove the top shell. If it comes with a new cable, take the time to install it, no matter how good the old one looks.
Once installed, connect to the Naza Assistant with the top shell off, and verify whether or not things are working.
If the answer is yes, you can obviously put the Phantom back together.
If the answer is no, you can take the gps out, and check it for physical damage, (cracked antenna, damaged wires, etc.)
Also check to make sure you have it plugged into the PMU V2, and not the Naza.
I think a new gps is around $165, so hopefully you won't have to go that route.
On a happier note: Your Phantom didn't fly away :!: