SQL Server 2016 DB is SUSPECT - cannot do anything with it!
The SQL Error Log message, "During undoing of a logged operation in database 'MyDB', an error occurred at log record ID (541478:273336:241). Typically, the specific failure is logged previously as an error in the Windows Event Log service. Restore the database or file from a backup, or repair the database." proved to be accurate, AFTER freeing up significant quantities of transaction log drive space.

Thanks to all who so swiftly applied their expertise in answer to this (never seen before by me) problem.