I don't have an Imac, so I don't know if there are specifics related to Imac.

If that was happening to me, first thing I would do is to shut down the computer, and restart it.  That should signal to computer to look for the printer. See if that helps.

If this is an on-going problem, I would look to see if I had the installation drivers (on a CD, maybe?) available.  If you don't have the installation drivers available on CD, I would look on the HP website, to see if I could find the installation drivers.  Once I know that I have them - or can access them on-line, I would: 

1) disconnect the printer (USB cable) from the computer;

2) uninstall the printer from the computer

3) restart your computer

4) reinstall the computer drivers onto your computer

5) reconnect the USB cable to connect the printer

6) restart your computer. 

I think I have that correct.  The instructions for installing the drivers should give you specifics on what to do.  

If you are having on-going problems, I would think uninstalling and reinstalling the drivers should address the issues.

Depending on how old the USB cable is, it is possible the cable is the issue.  Possible, but I doubt it is likely.  Trying a different cable would be an easy enough thing to do.  However, I think problem is more likely elsewhere.