The HP 33xx (specifically my 3330) software often loses the ability to connect to your printer/scanner, etc. I've found that killing off and restarting the background communication process sometimes solves this. Rebooting the printer can help too. Bundled are a python script to do this for you and an application that you can double click if you don't like to type at a command line. This simple app has been reported to work with other HP printers (model not mentioned) and is also useful when the process occasionally runs away eating up CPU and needs to be restarted.
What's new in HP Communication Reset
Version 1.1.1: hpkill.app wasn't working. Should be fixed now.
Version 1.1: Added a script and app to kill of the background communication process without restarting it. This results in less trouble with printing. However, you need to run hpcomm to restart it before being able to scan/fax.