This problem happens intermittantly when we try to print on the Canon Pixma iP8720 from our ASUS TP501U VivoBook Windows 10 64bit laptop. It has caused us all kinds of grief to correct over the past two years. It only happens on this laptop pc and none of the other pcs in our house. I finally worked out what I think is the quick solution to get the laptop printing on the iP8720.
When we get this message now, I turn off the laptop and the printer and reboot the wifi router. When the router is back up and running, I turn on the laptop and the printer. I then go into the Windows Control Panel > Hardware and Sound > Devices and Printers > Printers to check to make sure the printer is detected there. I noticed that we have been getting TWO icons for the Canon iP8720. The first icon is named "Canon iP8700 series" and a second identical icon right next to it is named "Canon iP8700 series WS". It is the first "Canon iP8700 series" icon that is the problem. It is usually made the "default" printer automatically but it does not work. If I switch over to the the "Canon iP8700 series WS" icon and set it as the default printer, the printer works again!
Don't ask me why this happens but it seems to be the solution to the "offline" problem. I pass this on to anyone else having this similar annoying issue in the hopes that it will be helpful. (Before I stumbled on this solution, I used to spend 2 hours laboriously uninstalling and reinstalling the software and configuring the printer to the laptop by USB cord. Now I have been able to get the printer going again with this solution in a matter of 5 minutes. Sheesh!)