This is just a "for your information" post regarding a problem with Canon printers that several people, with different models, seem to have had. I found a solution to my problem that I thought might be helpful to post.
No matter what I did, my Canon Pixma MG3520 remained offline. It worked at first and then entered this state after a while. At first I thought it was a wireless communication problem. I discovered that had that really been the case, the printer would have been described as "Not Found". Indeed, I could use the integrated scanner wirelessly. I tried everything, re-installing software, etc.; nothing helped.
I'm running Windows 7 64-bit. It turned out that for some reason both my printer devices in Windows (the wirelessly connected MG3520 as well as the same printer as an XPS device) were set to "Use Printer Offline". (If I'm not mistaken, that means that you would still like to print documents to the print queue for later actual printing when the printer comes back online.)
In Windows, in "Devices and Printers", double-click each device that is offline. You'll see a "Printer:" property set to "Offline". Select the property and it invokes the printer queue manager for that printer. On the "Printer" menu of that small window, there is the "Use Printer Offline" menu item. There will be a check mark next to the menu item indicating that this option is selected. Deselect it to bring the printer back online. You can confirm this with the previous "Printer:" property. Now it should say, "Ready".
I still don't know why this option got set in the first place. That it got set both for the normal printer device as well as the XPS printer device indicates to me that it probably wasn't accidentally set by me.
Hope this helps someone.
