It sounds as though it has lost connection to the base station. If it is BT Freestyle 2200/2500, try to re-register it. If you only have the one handset and it is not registered, it could be difficult to get the handset re-registered to the base station. DECT phones like to be "married" to at least one handset and this is usually done in the factory.
Hope this helps, Paul.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cordless phones.