Does your desk phone is no longer letting you dial out, or receive calls, or has a "Disconnected" indicator?  If the screen is on, but the phone isn't working properly, it may need to check back into the server.  It's as simple as unplugging the phone from the wall connection for a moment or so, and reconnecting it.  that causes the phone to reboot, and sign in.

However, if the screen is black: your phone is not getting power.  With that, you'll want to verify your cables are not disconnected, or connected to the incorrect ports.

The port in the wall, should connect to the port called "LAN".  There's a second port labeled "PC", that you can connect to your computer.  

 

If these solutions fail to resolve the issue, please get in touch with the help desk, and we'll get a tech out there to take a peek.