Try the following:

1. Reboot (nav will turn off 1min after entire car shuts down, when it powers up it has rebooted)

2. Load new OS (current is 4-1/100, fixes a zillion bugs compared to your version)

3. Clean the muck of your existing map DVD (you'd be surprised how much accumulates, makes it hard to read)

4. If you still get errors, consider new laser or new mech from the guy in the Netherlands. When lasers get old and weak, they struggle to read the disk. Crashes or 'no disc' are the result.

5. And please don't use chewing gum for anything