You should try a factory reset:
Step 1: Power off the HP Jetdirect print server.
Step 2: Hold down the TEST button.
Step 3: Apply power to the HP Jetdirect print server.
Step 4: Continue holding the TEST button for 5 to 30 seconds. Your JetDirect Print Server has now been reset to factory defaults.
Alternatively, the page here has some slightly different instructions.
In any case, I suggest you unplug and plug the ethernet cable to ensure it's in place. Even better, try with a different cable just in case.