We've had our Yale 6400 installed for about 4 years and it's been working just fine until today. My other half suddenly noticed that the doors weren't beeping when opened. Now we've realised that the remote keypad also isn't responding. The lights on the keypad are lighting up but it's not making any sounds. The main alarm panel seems to be working fine except that it's reset the date and time to 1 June at 00:00. The PIR's and sensors are still all programmed in but I can't get anything to respond. We've had no battery warnings and the main panel is charged. The alarm can be switched on from the main panel. We don't think we've had a power cut or anything. Any idea what's going on and how to fix it?