HP6978 printer has stopped duplexing

[hedera]

My default printer as it appears on Windows 10 is HP8C36BB (HP OfficeJet Pro 6970), but the actual device is an OfficeJet Pro 6978.  It also shows on my system as a 6978 which is "offline."  The default printer is networked. 

I recently had trouble with my scanner stopping working, and as a result, I had to reboot everything in this order:

1. Turn off the printer.  2. Turn off my computer.  3.Turn the printer back on.  4.Reboot my computer.  This got the scanner working.  But since then I cannot make the printer duplex when printing from my computer, although it duplexed regularly before.  When I look at the Device Settings it says that the Automatic duplexing unit is not installed.  And under Printing Preferences, the option to Print on both sides says None.

This printer was duplexing routinely up to last week, and suddenly I don't even have the option, printing from the computer.  But I can print 2-sided from the printer control panel.  What happened?  How do I fix this?

[hedera · Accepted solution]

I can't believe this worked, but I have duplex printing again.  Here's how:  After installing the Hp Full Web Pack, I noticed without much attention that I had a new device under Printers and scanners, called HP Smart Printing.  It had a Manage option, just like the others.  I opened that, and then the Printer Properties, and then Preferences.  This brought up a whole page of adjustable settings.  It was clearly talking about my default printer, same identifiers including the correct IP address.  Settings included Two-sided printing, which said None.  But it had a down arrow, which I opened.  I chose Flip on long edge and saved the change.  Then I changed the default printer to HP Smart Printing and now I can print duplex. 

I hope this description helps anyone else who may have a similar problem, but I can't believe the situation arose in the first place!  Talk about unnecessarily complicated!