How to fix "no route to host" problem

[Joe Lowder]

For many months, I have been using rsync to copy files
from one of my computers to two others, and it has worked
flawlessly. But today, it quit working and I cannot figure
out why and how to correct the problem.

Below is the error message I now see:

ssh: connect to host 192.168.0.4 port 22: No route to host
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(226) [sender=3.1.0]

What do I need to do to correct this problem?