How can I fix the ‘prohibitory symbol’ screen?
My mac tried to install an update, but it failed, and now it doesn’t start the OS,but shows a prohibitory symbol instead. Hard reboot didn’t help
What type of update did you do? That's been going around.
It means your machine could not find a valid macOS.
Different recovery options
https://support.apple.com/en-us/HT204904
Power off your machine. Hold down the option/alt key. Power on your machine. Continue holding down the option/alt key until you see an icon. Click on the start drive you wish to boot. Click on the right arrow icon or press the return key.
SMC reset
https://support.apple.com/en-us/HT201295
Do you have backup? if no, and you need to recover your data. See this link, give it a try but in your case I do not think this will work:
backing up from the command line via single user mode.
https://discussions.apple.com/thread/8101803?answerId=32357328022#32357328022
Or, you need to pull the drive out and place in external drive enclosure to recover the data.
Your post has
If you do not need anything on your drive, you can install a new copy of macOS.
-- option + command + r should work. power off. hold down option + command + r key. power on. wait to see icon.