Rear hydraulic brake failure.

2020 BMW C650GT, mileage 3025.
Just wondering if anyone has had a similar experience,and knows what to look for to start a repair process.
I had been riding for at least an hour,then came to a stop at a traffic light.
The rear brake handle went down farther than normal.
Just 2 min. earlier at the last traffic light it was normal.
At the next stop the brake handle went down MORE.
At the next stop the brake handle went down ALL THE WAY to the handle bar grip,and I had NO rear brake.
I drove home using only the front brake.
No sign of leaking brake fluid anywhere.
No sign of anything broken,no physical damage.
Just wondering if there is a similar experience out there,and where I should start repairing.
I just do not want to take something apart unnecessarily.

My guess is, if the fluid level in the reservoir is good and no visible leakage, that your master cylinder has failed internally, meaning the piston seal broke and fluid is leaking past it. I'd replace it with a good used one. Bench bleed it first before installing it. Maybe this is not the cause but that's where I would start. To check this, open the bleeder valve at the caliper (attach a hose first) and see if applying the brake lever makes any fluid come out. If not, then likely it's the master cylinder.

I found my brake fluid leak at one of the brake lines connected to the ABS Pressure Modulator.
I tightened the fitting and stopped the leak.
I created a new problem,I think by disconnecting the elec. connector going to the Modulator,
when I did the repair.
I now have NO working speedometer,odometer,or ABS. Brakes work,just no ABS.
I connected my GS-911 and it shows NO response from the ABS controller. The GS-911 software
shows NO ABS module. Is it possible to loose this from the scooters' computer just by
disconnecting the elec. connector to the ABS Pressure Modulator?

How difficult is it to change out the brake fluid?

Not hard, especially since you can apply pressure to the brake levers with one hand, and open and close the bleed nipple with the other. Supposedly the ABS pump has to be 'cycled' when bleeding the brakes but I've never done so without any ill effects.