1992 CADILLAC ALLANTE ...THE SPEEDOMETER CLUSTER NO LONGER ILLUMINATES. WHAT ARE SOME POSSIBLE THINGS THAT COULD BE WRONG?

Question edited for clarity, and add maker.
Question moved to make model category.

1992 is the closest

https://www.carfusebox.com/cadillac-allante-1992-main-fuse-box-block-circuit-breaker-diagram/

More here and on the images tab

https://www.google.com/search?q=1989+cadillac+allante+fuse+box+diagram

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If this is the analog intrument cluster and the entire instrument cluster is intermittently losing illumination, remove the instrument cluster light fuse and replace it with a new fuse. It is remotely possible that the actual fuse holder wires have broken due to age and would need to be soldered back together.
If this is the digital instrument cluster and the entire display is going dead, it could be anything from the wiring harness to the BCM. If it is the digital instrument cluster, I can really only recommend taking it to a reputable shop or Cadillac dealer to service.

Nope. The speedometer needs a signal from the vehicle speed sensor to operate. Is the check engine light on? If so, scan for any codes that may indicate a VSS problem. If no codes, possible internal cluster issue with the speedometer head

Here's a link to your code. P0050 HO2S Heater Control Circuit Bank 2 Sensor 1

The "cluster" means all of the things you see under the plastic cover in front of the driver, the 'gauges'. The term is a reference to the 'instrument cluster' including speedometer, tachometer, fuel gauge, idiot lights, temperature gauge, everything.
Get a second independent opinion since this is very unusual. Most likely you have a blown fuse.

I would wiggle the connector to the light switch it could just be a bad connection likely the master control for all the lighting

fuel cap bad or vent tube clogged with debris

try the spare key and if that doesnt work take it on a tow truck to the dealer

The Speed sensor found near the transmission was replaced. The part is not expensive, however the labor time & effort needed getting to and from the sensors location is the most difficult part of this solution. The digital speedometer seems to be working just fine now.

the speed sensor in the trans could be the culprit I would have the wiring inspected first though