Speedometer stopped working on my 1981 Kawasaki KZ550 LTD

My first post in this forum:
Speedometer stopped working on my 1981 Kawasaki KZ550 LTD. Bike has about 12056 miles on it. Bought it last December from original owner at 11027 miles. Well maintained and well kept, covered in garage with yearly maintenance records.
Odometer works, trip meter works, but just noticed yesterday during ride at about 12053 miles that speedometer stopped working. Knee jerk reaction was to order new speedometer cable which may arrive next week sometime. Haven't done work on a motorcycle in over 25 years, in fact this is my first bike in 25 years (kids, you know).
Why would the speedo stop working while trip and odometers remain operational. Any insights other than patience? Much appreciated, in advance

Your odometers remain operational because the driving worm gear for the trip and odometer is independent from the speedometer mechanism. Maybe the coupling drive train to the speedometer is no more functional. This could happen on old bikes in a split second......

BTW you have a really nice bike.

It sounds like something internal to the speedo. You can probably find one on ebay, or perhaps KHI still has original stock. I've used this guy many times and believe he does the best work of anyone on the planet, but not sure if he does non Z1 work.

zresto.com/

By the way, your bike looks really nice with original colours. My wife has the same bike but she girlyed it up a bit.

The speedo needle drive uses a magnetic system which gets its rotational input motion from a metal dome inside the speedo, driven from the cable. These drive systems get pretty flakey sometimes after 40 years, especially if the bike has sat around a lot.