What to do:
Look for the printer's icon in the dock (Note: you can also get to the printer queue by clicking on the Apple > System Preferences > Print & Scan and then selecting the printer.)
The icon is present in the dock and has a red circle with a number in it:
Double-click the printer icon.
In the popup window, select and delete all the print jobs; the user should be able to print now.
There is no printer icon in the dock:
Send the print job from the computer yourself (File > Print > select the printer, etc).
If it works, great, the user should be able to print now.
If it does not work:
Ask the user to move to a different computer;
Make a note of the steps you took and what did/did not happen with each step;
Report the issue using the "Report Learning Commons Problems" tab on the left.