Fault on the phone

Hello everyone! have recently entered into an office where the phone is handled by Asterisk.
A phone does not work for some days, I do not know Asterisk and telephony management in general.
I wish I could diagnose the problem but I do not know how. Can you help? Advise to do the test?
All phones work except one, the problem will be the single phone?
you can repair it or be changed?
if i buy new and difficult to configure?