AC 1200 R6120 not working anymore after update

Sounds like you  might have corrupted the firmware. we always recommend on here to update over a hardwired connection for best results. 

Did you try factory resetting? Push and hold reset for 7-15 seconds while powered on. 

If that doesn't help, you can try recovering it with tftp or nmrpflash.
are you on windows? if so, follow this guide
https://kb.netgear.com/000059634/How-to-upload-firmware-to-a-NETGEAR-router-using-Windows-TFTP
just make sure you enable tftp
https://www.thewindowsclub.com/enable-tftp-windows-10

and here's a video walk through using a tftp client. Use the built in one for windows 10 though. the video is more of an example of how its done.
https://www.youtube.com/watch?v=R9DLrLQrpaw

and if that doesn't work, you can try using nmrpflash
https://github.com/jclehner/nmrpflash

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mk654 wrote:
I started it and suddenly my phone said that i can’t do it.

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I don't understand that. What did you "start"? What was the exact message from the phone?

As plemans says, whatever you did seems to have broken the firmware.