I recommend reporting this as a bug. (This question also has some good information about bug reporting.) Reporting this sort of problem as a bug has the potential to help the Ubuntu project significantly.
What should I do when Ubuntu freezes? may help, but whether or not it does, reporting this as a bug is probably your best bet--even if it's not fixed, the troubleshooting that can take place in a bug report may lead to a workaround.
As you'll read (in the first two linked resources), effective bug reporting for Ubuntu requires identification of a package that you believe contains the affected (buggy) software.
Most of the time, when Ctrl+Alt+F1 has no effect, it is a kernel bug--sometimes a kernel panic (which is when the kernel "crashes" and has to stop, so the whole system stops working), sometimes other kernel or driver problems. I believe it is also possible for some graphical interface problems (with the X Server,
xserver-org) to cause this, though usually a GUI problem won't prevent Ctrl+Alt+F1 from working.
So, I recommend reporting this as a kernel bug. To do that, after reading at least that guide, run this command in the system where the problem occurs:
That will automatically send a large amount of potentially useful technical information to Launchpad, and will automatically open up a new browser window/tab where you can write your bug report, describing the problem in as much detail as possible (this is explained more fully in that guide).
In particular, I recommend including in your bug report:
- As precise an estimate as possible of when the freeze last occurred (so developers--and before that, triagers, if they wish--can easily find the most relevant lines in automatically attached log files).
- Whether or not Alt + SysRq + R E I S U B reboots the computer when the freeze occurs (and if not, what if anything does happen when you use that) -- you can always add that information to the bug report later, so there's no need to wait to report the bug until next time the freeze happens.
That won't be the only information you write in your bug report, of course.