I will update this answer as more information is gather and provided. Right now, as far as I can tell, this is the only kind of answer that can be provided. And it would be quite awkward to convey all this information in a comment. Also, since this provides detailed information about how to diagnose this sort of problem, I hope for it to be useful to others as well, and think that it therefore should be posted as an answer.
As izx has said, on a command-line system like Ubuntu Server, it's normal for the screen to turn black by itself after some time passes, if you're not actively using it. This is the virtual console version of a screensaver. Just press a key and it will come back.
Make sure your keyboard input is actually being sent to the virtual machine when you do this. If it isn't, that can create the impression that the Server is not responding, even when nothing is wrong.
If this is what's happening, there's nothing to worry about, but if you want to disable automatic screen blanking, see this question:
If that's not the problem, then more information will be required.
When the server is working properly (since it sounds from what you're saying that it runs OK for a while and then the screen goes black), SSH in from the host machine or some other machine capable of accessing it (install
opehssh-server first if it's not installed).
- To do this, run
ssh username@server-name-or-ip from a terminal.
- Or, if you're connecting from Windows and don't already have an SSH client, use PuTTY.
Then, when the problem occurs, see if the SSH session is still connected. If it is--that is, if you can still use the remote SSH login--then the problem is related to video.
If the connection is terminated or otherwise stops working, you might get some message. (If it's terminated you certainly will, though it may or may not be particularly useful.)
Whether or not the connection is Terminated, there are some logs you can look at. You can start showing them right after connecting, before the problem occurs. Feel free, of course, to use your own problem-solving skills and see which logs in
/var/log you think might be relevant. I recommend looking at the kernel logs:
/var/log/kern.log. You can view logs so that when new lines are logged, you see them immediately, by using a command like this:
tail -f /var/log/dmesg
Then edit your question to include whatever information you find. Using the above method, since you were SSH'd in remotely (though perhaps from the virtualization host), all text from the Terminal should remain visible (and copyable to the SSH client's clipboard) even after your virtual Ubuntu Server machine goes down.
So you can include some of it in your question, and could even include sections of basically unlimited length (including the whole thing) by posting it at http://paste.ubuntu.com (or just about any other Pastebin) and adding a link to it in your question.