Sorry to hear it.
(1) Have you powered down & back up at least 3 times? With real luck, it will boot & continue whatever it was doing with the update. But I guess going past 5 tries is futile.
(2) I guess I would try an HDD diagnostic after that. Turn on the computer & Immediately press ESC repeatedly for the Startup
Menu. Looks like F2 gets to the System Diagnostics...

If the HDD fails -- you will need to replace it. Then, it is possible you can restore your latest Windows system image backup to the new HDD. I've seen it said in various threads that will work. Otherwise, it's another clean install in the offing.
(3) What update was it -- the 2004 feature update? That one is optional, so you would have had to click the "Download & install" button to start it.
(4) Even if the HDD really is inaccessible, you should be able to boot to the Win 10 Installation USB drive & get to its recovery environment...
(a) Plug the Installation USB, then turn on the computer.
(b) Immediately press ESC repeatedly for the Startup Menu.
(c) Press F9 at that screen for the "Boot Device Options" screen.
(d) Select the 1909 USB Installation drive to boot to it.
(e) Click "Repair your computer" instead of to install.
This will reboot to the recovery environment.
(f) Then "Troubleshoot > Advanced Options > Startup Repair" is worth a try.
(4) Here is how to repair the file system, if it seems necessary...
"Troubleshoot > Advanced Options > Command Prompt", then...
*DIR C:*
*DIR D:*
*DIR E:*
You should recognize the Windows partition by its Windows, Users, & Program Files folders. Then use its letter in the following command...
CHKDSK C: /R <<< But use the letter you saw
That could take many hours to complete. It was 12+ hours for me first run. Subsequent runs were 3 hours apiece.