First thing to do is make sure you actually have unity and it is the issue. Then you must go into the terminal and launch compiz config, simply by typing ccsm(hopefully you can get into the terminal, other wise use the run dialog with alt+F2 and hope then try to launch the terminal from there). Then enable unity plugin, from there, it will most likely ask you to disable some other plugins so that the unity plugin will work, go ahead and do it.
Sorry if this doesn’t work and/or causes things to get worse. I can’t be held responsible for any of those issues and anything you do is at your own risk.
I’ve recently had a problem where I could not access the terminal. Switch to non-graphical mode by pressing Control+Alt+F1-F6, then type in
That is supposed to reset unity, so basically, fix it.