Boot hangs after installing the latest driver from PPA and Ctrl+Alt+F1 keyboard shortcut doesn't work

1. Immediately after the BIOS/UEFI splash screen during boot, with BIOS, quickly press and hold the Shift key, which will bring up the GNU GRUB menu. (If you see the Ubuntu logo, you've missed the point where you can enter the GRUB menu.) With UEFI press (perhaps several times) the Esc key to get to the GRUB menu. Sometimes the manufacturer's splash screen is a part of the Windows bootloader, so when you power up the machine it goes straight to the GRUB screen, and then pressing Shift is unnecessary.

   The timing when to press the left Shift key can be tricky, so sometimes if you miss it you need to try it again. If that doesn't work try the answers to I can't get the GRUB menu to show up during boot.

2. From the first purple GNU GRUB screen select Advanced options for Ubuntu with the ↑ and ↓ keys and press Enter. You will then be prompted by a menu that looks something like this:

   

3. Press the down arrow key until you select the second entry from the top (the one with the recovery mode in the description) and then press Enter.

   Now you should see this recovery menu:

   

4. Using the arrow keys scroll down to network (the third entry from the bottom in the Recovery Menu) and then press Enter.

5. Using the arrow keys scroll down to root and then press Enter twice.

   You should now see a root prompt, something like this:

    root@ubuntu:~#
   

6. At this stage you should have a read-only filesystem. You have to remount it with write permissions:

    mount -o rw,remount /
   

7. Uninstall the Nvidia driver.

    apt-get remove nvidia-graphics-drivers-*  
    add-apt-repository --remove ppa:graphics-drivers/ppa  
    apt-get update  
    reboot  
   

8. After you reboot the laptop, check if it is running properly. Then open the terminal and run this command to identify the right proprietary graphics driver from the default Ubuntu repositories to install.

    ubuntu-drivers devices  
   

   The recommended proprietary graphics driver for your Lenovo Y50 laptop will have the word Recommended after it.

I don't know what version of Ubuntu you are using or what the results of ubuntu-drivers devices will be for your laptop. The latest Nvidia driver in Ubuntu 16.04 is nvidia-375, but even if the recommended graphics driver is nvidia-346 or nvidia-current, it would be OK.