Is there a flashing question mark that keeps on appearing when you try to use your MacBook Pro? Make sure first that your MacBook Pro is turned off. You can turn it back on and press the R keys and the command keys on your keyboard. This will allow your Mac to start making a recovery.
You need to make sure that you will press both of these keys down until you see the Apple logo appear. You may need to do a startup soon so make sure that you are also connected to a reliable internet connection to push through with the rest of the process. If this does not work, you can bring your MacBook Pro to the nearest Apple service center.