OK, we need a little more information. When you "finish" booting, do you have a command prompt, or is it a completely blank screen? Depending on which it is, we go in different directions.
If you do have a command prompt, try typing "startx" and see what happens. You may need to do that using sudo, I'm not sure at that point, but I think you can do it from the prompt. If that works, then the issue is configuring your system.
If it is totally blank, your video driver is the likely issue. If you had a proprietary driver, for instance, it may not be working. In that case, try setting the driver back to a default: "sudo dpkg-reconfigure xserver-xorg".
Write back and let us know how it works out for you.