After some investigation I found that it looks like the setting is stored in the file
%AppData%\Local\Microsoft\HelpLibrary\HelpClient.cfg
Change this:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="helpmode" value="offline" />
</appSettings>
</configuration>
to this:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="helpmode" value="online" />
</appSettings>
</configuration>
And it should be using online help again.