Ubuntu

This is just a general question. I've had my computer lock up about a dozen times since I've been running Linux, usually because of multimedia plug-in type issues (but also because of badly-behaving math software). The screen doesn't refresh and I can't usually move the cursor.

What should I do in situations like this? In Windows I'd try Shift + Ctrl + Esc or Ctrl + Alt + Del. Is there something similar in Ubuntu? I don't think Alt + F2 works when it freezes like that.

You mean the graphics freezes not the OS?

You can try CTRL+ALT+F1. If that doesn't get you to a text console, there's something really wrong. If that works, please post the output of dmesg and the last few minutes of /var/log/messages

Clarification: There's nothing wrong with my computer now, I'm just gathering ideas for 'the next time'.

I thuink so, but I'm not sure to be honest. I'm talking about times I can't make anything happen: no mouse movement, no response to normal keyboard shortcuts (Alt + Tab or the like), but I can still see all the windows -- it didn't crash.

That looks good. I'll try Ctrl + Alt + Fx next time and see if it allows that.