This is really a job for a pro workshop and B&O should be able to
help you but if you are into electronics I'll be happy to give you a hint;
I don't have the manual at hand here but there's a transistor at
the extreme laser diode output from the controlling IC, a resistor
and a capacitor is in the same little circuit.
The capacitor is C2103. I don't recall the number of the
transistor or the resistor.
You need to measure the voltage at the resistor, a reading
from both ends of the resistor with respect to ground will be needed.
Alternatively, a voltage reading from the emitter and collector
pins of the transistor (also with respect to ground).
The circuit is found on the PCB that normally sits on the underside
of the CD module.
To make sure that the laser is on, all readings will need
to be taken while the CD is searching/playing,
so you need to mount the voltmeter, press start and monitor
the voltage, even if you only have a few seconds for each time play
is activated.
These readings will allow a further diagnose of the problem.
PS: I take it that a cleaning of the laser lens has already
been tried. Tapehead cleaner and cotton buds works fine.
Apply the liquid to a cotton bud and clean the lens.
Wipe off the liquid right away using a dry cotton bud.
Don't let the liquid airdry, it will leave behind a thin haze.
Martin