This is a common problem with USB printers and there are three main reasons it can occur:
To fix a print spooler issue is relatively easy and this usually solves the problem.
If you have tried to fix the print spooler as detailed above and you are still having issues, it may be the case that your USB cord or port is not working. Try a different USB cord or port and this should solve the problem.
If you've checked your USB connections, then you can sometimes solve persistent issues like this by resetting the entire print system, but please note: this loses all your printers & print settings, so it's quite drastic. Apple have a support note with the process here.