I'm not sure if this is the right place for this, but here goes ... after running this command: [ svn checkout svn://svn.mplayerhq.hu/mplayer/trunk mplayer ] the list was taking a while to populate, so I went off and did something else for a while. When I got back the command seemed to have stopped processing, probably because the computer went into screensaver while it was still running. I couldn't halt the command so I just closed the terminal.

When I opened a new terminal I was dismayed to find that all characters were now displaying as that empty rectangle that you get when a character can't be displayed. I decided to try restarting the computer, but that didn't help at all -- *all* characters now show up as rectangles. When I input a password and such it still works, so at least it should just be a display thing, though Firefox flatly refused to start up -- it must be very confused.

What do I do?

As you describe it looks like some fonts files are missing. If you weren't doing something else like an update or the like, it looks to me like your hard drive failing. In my (not so long) experience the drill is something like this:
-slow down the system
-spin the disk for longer periods
-begin to lose data <--(you might be in this stage)
-not booting meaning end function.

Try launching fsck /dev/[your device here] as single user or from a livecd if you have any.
Being [your device here] where you have the filesystem with the font files in.
after that type "echo $?" to see the exit code; 4 means errors not corrected 8 means operational error (see man fsck)

It happened to me in some (around) 3 years old drives but i've been told it might happen earlier, even during warranty periods.

Good luck and Backup your personal data

Come to think of it, I believe I was doing a small update when this occurred. Could that be causing the problem?

I'm pretty sure that it's not the hard disk, as I got it new only a few months ago and have never had any problems with it. Fsck returns 0. I noticed that when it boots I seem to be getting some warnings I usually don't -- I don't remember what's normal well enough to say which, though I did note the line 'Filesystem is NOT clean.' -_<

I checked the font folder and it seems okay, though it's not like I know what should normally be there.

Assuming that you did NOT run svn as root, but did run the "small update" as root, yes, that should be the problem.

Sorry, but that doesn't make sense. When my hard drives start to fail, they were just working right before they started to fail. They always work right before signs of failure occurs, otherwise you had not installed them. The chance is small, but it is way too early to preclude it.

If nothing else, it become time to study these warnings very carefully. Do dmesg | less to read them in your own pace. Check also /var/log/messages.

Again, this doesn't tell anything. A directory listing gives the contents of the directory, but doesn't indicate whether the files are still uncorrupted and present.

Tell us what update you made, post your distro, and post any warnings by copy & paste, so we see them verbatim here.

jlinkels

Yeah, you're right, of course. -_- Well, I only got the errors during the one boot for some reason, so the filesystem is returning clean again now. I'm using Linux Mint Gloria. The update I made included a bunch of pulseaudio stuff, mint-artwork-gnome, mintnanny, mintsystem, mintinstall, mintmenu, and mintupdate. Obviously, I should have been paying more attention. At the end of the update log it says it reloaded /etc/samba/smb.conf.