HP CLJ 4700 empty display, no answer

An interesting problem with no solution... If I switch on our HP LaserJet 4700N, it sounds everything is OK, I heard that it check toners as usually. But the screen is empty. It is lighting, but nothing text. I can't reach the printer over network nor with usb.

Can I make some reset or something without the display? I mean like some older printers, pressing a button while I switch on printer or something like this.

Thanks for Assistance.

When the display is light and printer is ready for printing but does not pass pages:
There are still jobs in the memory buffer of the printer.
This could be solved by pushing the oval select-button.

1. Entering the oval select-button also activates the new control panel settings (when they have been changed).
2. Printer control panel settings could have been changed over a network cable: checking the settings is a next step when problems do