Hope this does not happen to anyone else. I am posting my solution in case someone needs it.

background:
I was debugging a problem with a video board and in the process lost OpenCL and Windows failed to boot and tried a repair
I got window working after removing the defective video board. Obviously, this caused problems.

BOINC was unable to process any work units requiring OpenCl. The log shows that OpenCL was available for the CPU and the Intel GPU but not for the Nvidia boards. However, the log showed that CUDA was still available and had the correct version and date.

Running clinfo.exe showed the exact same thing as Boinc did: OpenCl only for the Intel CPU.

I did a re-install of Nvidia driver 526.86 but that did not fix the OpenCL problem. I then got the latest 528.02 Nvidia and did a clean install but that did not fix the problem either. Googling I read that windows\system32\OpenCL.dll was where all the good stuff is kept. That Nvidia version was dated 2016. i replaced that dll with Nvidia OpenCL.dll dated 2022 from another system and rebooted. OpenCL is now working fine.

Unaccountably, an install of latest Nvidia driver did not update that DLL.