I used Startup Manager to edit Startup settings and when I restarted, I got a black screen and couldn't log on. Please help.
I think you messed up the boot settings by using startupmanager. Use the steps mentioned above and trying booting into the rescue mode.
Step 1: Find out what's happening
You (and we) need to know what's happening to have a chance at fixing it.
Hold the left control when it boots. Grub should load up, giving you choices of what to boot. If it doesn't or there aren't any choices, we have a problem so let us know.
If you do get a choice, you'll see a choice to edit. Do that on the first Ubuntu option and in the next screen, edit the boot command so
Then control+x to boot. Let us know what you see.