IS300 (2002) Satnav cannot read disc

[br1anstorm]

The satnav in my 2002 IS300 Sportcross seems to have developed a problem.  Until recently it was working perfectly.  Now when switched on it starts as usual - briefly showing the Lexus logo, then the menu-screen with instructions and language and the "I agree" button - and then (for a moment) the map.  Then up comes an error message saying "DVD - unable to read data.  Check if map DVD is correct".

Attempting to access other menus or set a destination using the central-console buttons sometimes brings up the relevant screen, but after a few seconds the same error message appears.

I have ejected the (original and genuine) DVD from the unit beside the spare wheel under the boot floor.  It looks clean and unscratched, and the whole area is clean (no damp or dust);  I know the car's history and the wiring etc is all original and unmodified.

I've cleaned the DVD surface;  and I have also used a lens-cleaner disk in the machine itself.  That hasn't cured the problem.

What other checks and solutions might I try .... and in what order?  I have seen it suggested on some forums that (a) disconnecting battery-power and (b) unplugging the wiring to the unit in the boot, waiting a minute or two, and then reconnecting, might help.  I assume that is equivalent to a "reset" and it wipes the stored data.  But that must be saved somewhere else (on a chip or card) separate from the DVD, so it's not obvious that this is a relevant step.

Could it be that the laser which "reads" the DVD has simply failed suddenly?  How can this be checked or tested? 

Any other explanations or suggested solutions? It's not a very sophisticated satnav (no postcodes...) compared to current models, but I still find it very useful, and want to fix it if possible.