As of 8:30 p.m. today (18 Jan 2018) an update was installed to WhatsApp and now everything is working. It restored messages and contacts that were saved to OneDrive. It was necessary to reconnect computers using the web app version. I have no idea what the problem was, or what was done to fix it, but at least for the moment it is working on my phone, which is a Lumia 640 running Windows 10 Mobile, Version 1709, build 10.0.15254.158.
Hope that is helpful, Ron