Computer I am trying to attach to shows as OFFLINE

I am trying to attach to a remote PC. It is my mother's computer - she's 89 and needs help periodically.  I have attached successfully in the past.  Today I attached but she was using it, so I detached.  I did that three times the past two days.  I just tried again, but I received a message that the computer I was trying to attach to was offline. 

I got it fixed by asking her to restart the program at her end.  Got a new password and was able to get attached. But this should not be necessary, unless I am reading the documentation from TeamViewer wrong. 

Can anyone tell me why this might be occuring, and why do I seem to need to get a new password from the computer I am trying to attach to?