ok dont worry i have the solution for you! your systems fine and before you take it and your whole car apart- dont bother! i will save you 6 hours of worrying!
as you can tell i have had the same experience and tried everything i could think of before i finally worked i out. this was a few weeks back in sub zero temperatures at 2am, im sure the neighbours were on the verge of calling the police thinking what the hecks this guy doing in the boot of his car at this time!!!!

anyway what has happened is your drive has gone into continuous loop mode. these drives are very sensitive and once they dont like a disc, they will go into this mode of continuusly trying to load the 'bad' disc.
the solution is very simple but you have to be very patient(something i know is very hard to do when its freezing and you've been at it for 6hrs!)

the easiest way to do this is:
first make sure you are in a very quiet area, you need good hearing to do it this way. put the cd changer cover/flap thing down for easy access to the nav drive.
lock everything and wait at least 60secs- (to make sure all systems are shutdown)
go to the boot, unlock the car and very quickly open the boot and hold the eject button on the drive.
keeping this pressed and with your head as close to the drive as possible, listen for the disc spinning,
after approx 2-3mins(there is no exact time) you will hear the disc come to a stop, at this point release the eject button and than quickly press it again. your disc will pop out and the drive will come back to life.
Note: you will hear the disc sound like its come to a stop 4-5 times before it actually stops, dont release the eject button yet though. this is just when the disc slows down, you must wait to the actual point when it stops completely.
this is the only 'window' of opportunity for you to eject the disc, other wise if you do it too early or late, nothing will happen and the disc will continue 'looping'.

your probably wondering how i worked this out!- well there is some information out there on the net but no-one explains in detail, they just give approx times and it does not work like that, believe me i tried everything for 6hrs before i finally worked it out myself.

the way i personally done it was to remove the drive from the car and remove the outer cover so i could see the disc(there is a manual way to remove the disc like this but its very delicate and you could easily damage the unit. so with the cover off i plugged it back in and observed the actions of the disc, i noticed it only completely stopped at one point, between 2-3 mins after initial start-up. this was the million-dollar moment!

if you cant time it properly by putting your head by the unit, than you'll have to do it this way, by watching the disc physically stop and than releasing and pressing eject.

i know this is a long post but i can guarantee this works, i found out the hard way!

Good luck and let me know how you get on

Cheers