I am guessing that when you installed the proprietary driver, Unity selected a resolution beyond your monitor's maximum resolution. That is why your monitor says Can't Display. Therefore all you gotta do is just change your resolution!

For this:

• When you are in that Can't Display screen check if you can hear the login sound.
  This would tell us if you are in your login screen.


• Then type in your password and press Enter.



• Now we will assume we are in the desktop screen. Now try opening a terminal(with Ctrl+Alt+t).
  
  Then type the following command:

  
  
  

  xrandr -s 1024x768
  
  

  
  
  

  I am assuming 1024x768 is a supported resolution in your monitor. If its not supported then type some other supported resolution. This should show your desktop.

If this doesn't work then I suppose you can just delete your proprietary driver. Since you say that your purpose for installing the driver is just to get your maximum screen resolution detected you can just delete it and get your maximum screen resolution detected by following the steps in the answers whose links are as follows(The first link is better):

1. How to make my maximum screen resolution to be detected by Ubuntu?




2. How can I get my monitor's maximum resolution without the proprietary AMD graphic driver installed?