Debian (and Ubuntu, as its descendant) permanently binds the ethX network interface to the corresponding network card's MAC address, so after replacing a network card or its MAC (for example, in the case of a virtual machine interface) in Debian, it happens that the corresponding ethX disappears. For example, when you replace the first network card, eth0 will disappear. Even if only one network card remains in the computer - it will no longer be eth0, but eth1, eth2, etc.
If there were two network cards and you replaced the second one, then instead of eth0 and eth1 you'll get eth0 and eth2.
Fixing this is simple: after replacing the network card, you need to boot up and go to the file /etc/udev/rules.d/70-persistent-net.rules , where you find and remove the card that is no longer there, and change the eth name for the newly installed card.
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