LEGACY R SPEC B - BLANK NAV SCREEN/ SECURITY. HELP PLEASE

[NOACROSS 414]

Hi All-

Have been driving the above 58 Legacy for the last year, no problems.  Decided to sell it reluctantly, and on picking it up from the workshop after MOT and service, the Nav screen (radio etc) is now locked out. 

The screen is just blank except for the word 'Security' and no matter what buttons I press or how long I leave it, nothing changes.

Any ideas? Customer obviously wants his new car soon.