My computer says my printer is offline. How can I fix that so my computer will tell my printer to print

I too had "printer offline" messages on a regular basis after one of the recent macOS updates. Previously the HP printer had worked well for me for several years. After trying HP and Apple updates & recommendations, and others that I found on-line, still no joy. My almost daily go-to fix was to shut the printer down and reboot it. I was printing using Apple Air Print and connecting via wifi. I finally decided to try the hard wired approach and came across an appropriate USB cable before an ethernet cable so I went with that. Bongo ! It worked right off the bat and has continued to do so ever since, no rebooting required.

Sometimes it helps to go into the printer settings, delete the printer queue and re-add it.

Add a printer to your printer list so you can use it on Mac - Apple Support (VN)