When you did the Ctrl-alt-F2 what you happened was that you switched to
another of Linux's desktops, a command line one. Only one runs the nice
graphical desktop: it's Ctrl-alt-F7, desktop number 7.

While it didn't exactly say "enter username" thats what it wanted by
prompting with "login". When you enter it and press the "Enter" key then it
prompts for that user's password. To get back to your proper graphical
desktop it's Ctrl-alt-F7.

The best way to get a command line from your normal desktop is Alt-F2 (not
Ctrl-alt-F2) which just gives you one line to type in your command - no
login. If you get there by accident there is an "X" in the corner to close
that little window. Try typing df -H in such a window. It tells you how
much space is free in all your mounted drives: d for disk, f for free
space, (space) -H for show it in human readable form.

There are many, many fast and handy tricks like that, but just forget them
for now. When you are more familiar with Ubuntu you can gradually
accumulate a list of the ones you like and keep it by your keyboard.

I always told my students that using a computer is not rocket science. It's
just that you have to get a lot of little steps correct for it to work at
all. If you don't have every one of your ducks in a row it can be extremely
frustrating as nothing works. Computers have absolutely no sense of humour,
and usually wont help you correct your mistakes. Linux, especially the
'buntus, is much more helpful than Windows, but it can't work miracles...