The question mark folder means that the computer can’t find an operating system to boot too. Sounds like either the update failed and corrupted your installation or your hard drive died during the update. You can try holding command r while booting to boot to recovery and see if you can use disk utility to repair your disk. If that is unsuccessful, you can try erasing your drive and reinstalling macOS if your drive is still good. If you have data you need to recover, you can boot the Mac into target disk mode by holding t (Before you wipe the drive) and using a firewire or thunderbolt cable to connect it to another Mac and copy off the data.