One of my Raspberry Pi’s that performs sump pump depth measurements every 5 minutes works headlessly. Last week it stopped supplying values via the network to my other Pi, which does the reporting via a web page.
So I tried to reach the Pi via putty. Putty responded with “network error
connection refused”. This led me to checking cables, and I found that everything
was fine. To make a long story short, it turned out that a recent power loss
must have caused a corruption within the networking setup of the Pi.
I hooked up a monitor, keyboard and mouse and did a “sudo apt-get update”
followed by “sudo apt-get upgrade”, followed by “sudo apt-get
upgrade --fix-missing”. Lastly, I reset the SSH keys by issuing “sudo rm
/etc/ssh/ssh_host_*” followed by “sudo dpkg-reconfigure openssh-server”.
After this, things were back to normal.