My Mitel 5340 ip phone will not turn on. i've changed the cable's and it still won't turn on. what could it be?

Login to voicemail and change it!

Contrast is turned down or bad display

if you go to voicemail then it will say hit "u" for user options, then hit "p" or "7" to change passcode.
i just did that(:

The network isn't configured properly for your phone to boot up. You should contact your phone vendor or system administrator for support.

It means that your DHCP server on your network is missing the options to allow your phone to boot up. After a month (time on this message), I hope you've managed to get the issue resolved. If not, I hope you've fired your IT support team - they should be able to get this resolved quickly.

Sincerely,
Mark

The mailbox will need to either be defaulted or at least the password. Either your on-Site system administrator, or your vendor will have to make the change.

good luck,
Mark

Are you referring to the Mitel Cordless handset or headset? The handset and headset must be paired to the module on the back of the phone. Press and hold the button until the display indicates pairing. Then press the Answer button on the handset or headset.

You need to set this extension up in the Call Rerouting form in the PBX. Program the First alternative to go to the voicemail extension.

I believe the room phone is off hook.

The only electronic hook switch that I know about is on the 5330/5340 series phones, with the cordless headset from Mitel.
Here is a link to see what they look like... I have several installed at sites and the customers love them, no more handset lifters are involved. http://www.mitel.com/DocController?documentId=25305