Flashing question mark folder

Have you tried Internet Recovery? Hold option-command-R together at power up and make sure you're connected via ethernet or that you can access WiFi that doesn't require a login (a WiFi password will work).

The flashing question mark on a folder indicates that the Mac is unable to find a system file on the disk drive it can use for booting. Try doing an internet recovery boot and check the disk drive using Disk Utility.

To me it looks like your recovery USB flash doesn’t contain proper macOS Installer.

You have two options if you have any kind of network accessible Ethernet or Wi-Fi, use Internet Revovery with ⌘+⌥+R right after you press power button.

Or if you have another Mac available make new Recovery USB following this article:

https://support.apple.com/en-us/HT201372

Let’s see if you can boot up at least into macOS Recovery. Then go into menu, start Terminal and run ‘diskutil list’ or ‘diskutil apfs list’ and post output here.

Good luck...

Hi, What happens if you hold ⌥ (option) key while powering-up? With USB Installer present in the port.

MarkLase wrote:

Problem was in key that stuck. I clean it and now my computer works. Thanks all for helping

Which key? I guess it might have been forcing some alternate startup mode.