I had this issue too.
For me, it was actually quite easy to fix, and there’s no need to reboot.
You must edit a Compiz-Fusion setting:
Open the following:
System → Preferences → CompizConfig Settings Manager
If it isn't installed, go to the terminal and run this command to install it:
sudo apt-get install compizconfig-settings-manager
Scroll down to the Effects section, where you will see that "Window Decoration" is not enabled.
Check the box to the left of it, and you should see your window borders appear again.
Like i said, this is the solution i applied for this issue. Hope it helps, for you too.