I may have solved this. This is my daughter's laptop and she was running Steam (game engine) as a 'Login Item' at startup. I suspect, given the pattern of the non-working keys, that a game loaded with the engine and inadvertently disabled/interfered with the built-in keyboard.
I removed the startup item from the list, rebooted, and the problem was resolved.
1. Open 'System Preferences'
2. Click on 'Users & Groups'
3. If needed, click the lock to make changes (lower left).
4. Click on the 'Login Items' tab.
5. Identify any software at startup (e.g., games, game engines) that could interfere with the keyboard.
6. Click on the item(s) in the list and then click the minus ( - ) button to remove them.
If I run Steam after startup, like any other application, the keyboard continues to work as expected.