This happened on my 3B+ when I first started it. The cause in this case was that the ethernet cable was not plugged in properly.
My fault as the cable I was using had previously been plugged into a television and I had cut off the locking tag to make it possible to get get the cable out without removing the television from the wall.
From one of the replies I went to find the linux equivalent of ipconfig /release etc for windows. I found the answer and was about to try it on the Pi Zero when my network went down.
Here I should say that I am or was using a VM Hub 3 in modem mode with a TP Link Archer AC 1750 router. I had this setup for about 2 months and everyhing was working, but yesterday the problems began after I rebooted the router in an attempt to get a DHCP IP address of 192.160.0. something.
My IP cctv cameras were not showing anything on another monitor, but a switch showed that they were working,from the blinking leds on a switch. Then there was no wireless connection to my android equipment.
So , in conclusion, I disconnected the TP link router and set the vm Hub back to router mode, and after a short while , my network came back, cameras were displaying pictures on the monitor and wireless was back.
So for now, I am going to stick with this setup, and I have no ideas as to why the TP Link router developed the said problems.
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