If your Debian system freezes up, it is possibly a bug and the system has crashed. However, it's fair to say that this doesn't happen very often --- many people go for months or even years without a crash, even under heavy use. GNU/Linux is extremely reliable. If you have problems more often than this, something is probably wrong; ask for help (see Getting help from a person, Section 5.5).
However, it is much more common for the screen to freeze due to a bug in the X server, especially when switching to and from the X virtual console. The newer X servers are more likely to do this, or simply crash; it's hard for the XFree86 project to keep up with the latest features of every video card available.
If the screen freezes, the system is usually still alive, you simply can't interact with it from the console. If you're on a network, you should be able to log in from another workstation. Once logged in you can reboot in an orderly way (i.e. type reboot). If you aren't on a network, pressing C-A-DEL will work most of the time. If neither of these works, you may have to simply shut down the system and let fsck repair the disk on your next boot.
Sometimes you terminal will get stuck in reverse video mode, will refuse to move to a new line when you press return, will print garbage characters, or otherwise not work properly. This is almost always the result of attempting to view a binary file with cat or more (sometimes it's also caused by a bug in a program). You can try to recover by entering the reset command. If it did nothing, try typing echo C-vESCc (ie: press C-v, then ESC, then 'c'). If that fails, log out and back in.
The problem is that terminals are controlled with special control sequences which tell them to enter reverse video mode, position the cursor, etc. If you send a binary file to the terminal, the file may happen to contain these control sequences. Thus the terminal will get confused.
You've inadvertently gotten stuck in vi. See Creating and editing a text file with
vi
, Section 8.3.
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