I haven't seen this a lot, but it's common enough that you can find it in a Google search. Here are some of the top results:
- HP 7520 - Printer in Use - won't print from MAC
- I have a new iMac running OS X 10.8.3. I am adding a Windows network printer by IP address. The iMac sees the printer, but when I print it says "Printing-The printer is in use." and hangs forever.
- unable to reset: "printer is in use"
I recently came across this situation with an HP printer, and I thought resetting the printer system was a bit extreme (especially since the user who brought it up had about six or seven printers installed that would have to be reinstalled).
I tried removing the printer and re-adding it. Same problem.
I tried adding the printer to my computer (to see if it was a problem with just the user's configuration or if the printer itself was having issues). I got the same problem on my computer, too.
I went through the HP menus to print out the printer configuration page. It spit out a backed-up print job before printing the printer configuration page, but not all of the backed-up jobs came out, and the "printer is in use" error persisted for both me and the user I was trying to help.
I waited a bit and then saw a message on the printer to hit the checkmark button. I hit it again, and all the backed-up jobs came out and the error went away for both me and the user.
So I don't know if that will always be the solution, but I'd at least give that a try before resetting the printing system.