i upgraded my Macbook Pro to Sierra and now have a black screen with a white folder flashing in the middle of the screen with a question mark on it

tania212 wrote:

Why has this happened and how do I fix it

According to: About the screens you see when your Mac starts up - Apple Support

"Folder with a question mark

If you see a folder with a question mark appear instead of the Apple logo, it means your Mac couldn't find a local or network-based startup disk. This can happen if the disk selected in the Startup Disk pane of System Preferences isn't available. Wait a few seconds to see if your Mac is able to locate the startup disk you specified.

If you still see a question mark after waiting a few seconds, use Startup Manager to start up your Mac, then select your startup disk from System Preferences.

If a question mark appears after you install a software update, select your startup disk again using macOS Recovery."