Can you get to your login screen? On it, you should see a power button on the lower right corner of your screen. If you click it to bring up the choices (Shutdown, Restart, and Sleep), and then hold down your Shift key while you click Restart, it should restart in a troubleshooting mode. You should see a screen titled "Choose an option". One of the options is Troubleshoot. If you click it, you'll see more choices; one of those is "Advanced options". If you click it, one of options on the next screen is System Restore (restore to a Restore Point).