Ubuntu sometimes puts the X session on console #2 instead of #7. Try alt+ctl+F2 if this happens again, and see if that's your X session. For that matter, if it isn't #2, try 3 through 5. If you would like to have regular non-X consoles on 2 through 6.
In your /etc/inittab file you'll see something like this:
# Note that on most Debian systems tty7 is used by the X Window System,
# so if you want to add more getty's go ahead but skip tty7 if you run X.
1:2345:respawn:/sbin/getty 38400 tty1
2:2345:respawn:/sbin/getty 38400 tty2
3:23:respawn:/sbin/getty 38400 tty3
4:23:respawn:/sbin/getty 38400 tty4
5:23:respawn:/sbin/getty 38400 tty5
6:23:respawn:/sbin/getty 38400 tty6
I changed the lines that begin with "1:2345" and "2:2345" myself, because I wanted a console in addition to number one.
The 1:2345 means console one is available for run levels 2,3,4, and 5. The 2:2345 means the same thing for console two.
Kind of confusing but if you will read /etc/inittab it goes into some detail.
Just be careful with typing when modifying that file--or just about any file in /etc. It's a good idea always to save a file you're about to modify, under another distinctive name such as "/etc/inittab.this_one_works" just in case a major error occurs and you need to restore it in order to use your system.