Hi & Welcome to the site Jinny.
Sounds to me like either the base unit has had its power supply interrupted or the phone itself isn't registered with the base unit (someone else being playing with them possibly?

)
What is it showing on the LCD display on the phone thats got the issue?
Should you need to re-register the phone with the base unit again then see
here for the procedure (taken from the Instruction manuals) to register phone to base unit for the d5100 set.
Hope this helps.