I want to share the following solution because I did not find it on the internet (yet).
Luckily I had made some notes of commands and thought this one might work.
After a power-dip I lost my IPv4 connection on my OpenHab running on a Raspberry Pi.
First I thought the dip bricked my card but then I noticed the IPv6 connection was still working.
On Internet I did not find my solution, only a temp solution valid for a single session to activate IPv4:
sudo dhclient -4 -v eth0
Long story short, the solution was simple, remove any existing leases on the PI and reboot:
sudo rm /var/lib/dhcp/* sudo reboot now
Possible Root cause(s)
Apart from the powerdip, I changed 3 months ago ISP/router (can not remember I needed to reboot PI). The later ISP was cable (ZIGGO) where I used a reserved DHCP in the router for the PI (so no static settings).
It can be easier (in Putty) to access your device with its IPv6 address
Release = Raspbian GNU/Linux 9 (stretch)
Kernel = Linux 4.19.66-v7+
Platform = Raspberry Pi 3 Model B Plus Rev 1.3
OpenHAB 2.5.10-1 (Release Build)
Hope this is useful for others with similar problems