Map DVD has stopped working

Any scratches or dirt on the disk?
It's possible there's dirt on the lens used to read the disk too. Sometimes I've had luck at fixing optical drives by blowing compressed air into the slot. Of course, that just blows the dust around, so it might make it better or it might make it worse. So I'd recommend replacing the disk (or at least borrowing someone else's) first to make sure. If it's not the disk and your only option is to replace the nav system, blowing some dust around is a lot cheaper and easier, even if there's some risk of making something worse.

 

Optical disks read from the center of the disk first, so make sure the disk surface at the center portion is clean and scratch free.

 

You already said you took it out to try and fix it!

Unlikely, but it would be possible for it to get scratched while going over a bad bump, jostling the pieces inside the optical drive or the disk itself so that something scraped. Random dust or some other type of failure is more likely.