could be several reasons,
printer not installed correctly, snmp not working, tcp ip port changed, bad driver??
have you tried installing the latest global driver? on the download page there is a webinstaller that installs a new driver for you :)
We are having a problem with printers saying "printer is unavailable". I've looked in the properties of the printers and do not see a tab for troubleshooting. Can you tell me what is causing this message to show up and how do I fix this/