Account was locked and I don't know why

I don't use UBER very frequently. Maybe 2 or 3 times a year. Last fall, I received a notice that I had a "poor rating", althought I don't know why. None of my rides had issues that I know of, and there was always pleasant conversation AND I tip well. I dismissed the notices as some mistake or maybe they aren't happy I'm an infrequent customer. Saturday, I got a ride and had a great talk with the driver. When I got out of the car, I tried to tip the driver, and my account was locked out. I didn't even have an opportunity to tip. I assumed there were technical issues....but now that I've raised issue they refuse to reinstate. AND they still won't give me any specifics. Honestly I'm stumped. I'm probably among the cleanest and easiest riders they have! Now I feel bad for that driver. I even received a remider email 12 hours later asking that I rate the ride!