I've encountered this frustrating behaviour quite a few times when supporting my clients. The most recent scenario was with a Dell laptop and a Brother label printer. Both the laptop and the printer were turned on. The Brother label printer was connected to the laptop directly using a USB cable, but the printer properties still showed that the printer was 'offline'.
Assumptions:
1. The printer drivers have been installed
2. The printer has been working and printing fine prior to this issue.
Solution 1 (USB Connection):
This quickest solution (if it works) is to unplug the USB cable from the printer or the computer, whichever is the easiest to reach. Then after a few seconds, let's say 10 seconds to be safe, plug in the USB cable back in. If the drivers have been installed correctly then the computer should recognize the printer and it will switch from offline to connected!
Another version of this is to turn the printer off and on again. Assuming you can reach the printers power switch, turn the printer off wait a few seconds (10s) then turn the printer back on. If the printer doesn't have a power switch then you would unplug the printer then plug it back into the power outlet.
Solution 2:
Sometimes the printer has been manually toggled into offline mode using the printer queue manager. If this is the case then you have to open the printer queue and from the 'Printer' menu click the option 'Use Printer offline' to toggle the printer online. I've illustrated the steps below.
Click on the Windows 10 search box or press the Windows key on your keyboard. Start typing 'printers...'. Windows will suggest docs and applications that match what you are typing and you should quickly see the Printers & Scanners System Settings option appear.
From the list of installed printers, click on the printer that you need to fix. This will display the options menu.
Now click the button labeled 'Open Queue'
With the selected printer queue open click the printer menu item (1). If you see a check beside the Use Printer Offline, click that option to switch the printer from offline (not working) to online (working). You can try printing to the printer now. If the printer is printing properly again congratulations are in order, you've finished troubleshooting your printer.
If there isn't a check beside the use printer offline you'll have to repeat this step twice. The first time is to switch the printer offline. This tells Window to stop communicating with the printer device. Repeat this step a second time to turn the offline mode off (yes this is a bit confusing). This tells. Windows to start communicating with the printer device again.