EL52251 handset mystery Can you help me?

I have a 2 handset EL52251 telephone system. Both had been working perfectly til about an hour ago (as far as I know) when all of a sudden I see that Phone #1 is displaying "1. Press Find HS on Base for 4 sec." followed by "2. Then Press # on handset." So I did. Then the phone tells me it's "registering" (What for? I ask myself. It can't vote.) Then it beeps twice and says that registration has failed. A little internet search and I am now wondering if the darn thing has unregistered itself from the base (and I wonder, "How? Why"). And more importantly I want to know how to make it reregister itself so I again have two working handsets. 

Can anyone help me?