Firstly you should check the cabling, it is normally someone unplugging a phone or patch lead that causes the problem. If that looks OK then try changing the phone, it's not often that they go faulty but it could be worth checking.
Sometimes the phone lead can be run over with a chair or desk, that can break the wires inside.
Last resort is to end and reinitiate the extension, it might just have locked up, but this is doubtful.