Hi,
There are few items to check
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You need to dump the log as linuxdev shared in comment#2 so that we can find out what happens.
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There is a known issue for jetson-nano-2GB device to have blanked screen after running apt-get update/upgrade. This is the thread.
But since you said the monitor is not showing anything even before update, you should still dump the log for us to check. I mean you should restore to pure/initial image which is before update and share us the log.