I finally have the solution to the problem...without knowing the exact problem! Here is what I did, which fixed the problem.
In the little Cortana search box in the lower left corner of my Windows 10 desktop screen, I typed in "Network Reset" (without the quotes).
A new window opened, and I clicked on the "Reset Now" button.
The computer needed to be restarted manually, even though the screen said it would restart automatically. So, I manually restarted the computer, brought up the www.testmy.net website, performed a manual 25Mbps speed test, and I'm back up and running at 25Mbps+ again.