Flashing question mark
AFter upgrading OS, it restarted and now I can't turn on computer, help!
That means your computer could not find a valid macOS on your machine. There have been a rash of these reports when people upgrade. I do not know of a definitive reason for this nor a definitive fix.
Id say that your filesystem has problems. I'd try to get into safe mode. Safe mode checks and repairs the filesystem.
Try a safe boot.
Shutdown your machine. Hold down the shift key. Poweron. The boot up will take longer than normal because the filesystem on the startup drive is being checked and repaired as needed. All about safe mode including what features and apps safe boot leaves out.
http://support.apple.com/kb/ht1455
Do you have backup? if no, and you need to recover your data. See this link:
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 the machine stopping in the middle of a boot.
-1- Safe boot may work. power off. hold down option key. power on. wait to see icon.
-2- single user mode may work power off. hold down command + s key. power on. wait to see icon.
-3- command + r should work. power off. hold down command + r key. power on. wait to see icon.
-4- option + command + r should work. power off. hold down option + command + r key. power on. wait to see icon.
try in this order report the results for each.
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