Black screen during boot after TTY1 login

Black screen during boot after TTY1 login

I've got a fairly fresh install of 13.04 (3.8.0.29) and it's been running
fine for days in a row. I turned it off for the night and now it won't
boot -- or at least, it won't show me anything on the screen.
This is not an Nvidia problem, as the machine has a Radeon HD 5450 card.
IIRC it's running with fglrx (not -updates and not built-in because then I
wouldn't get my 1280x1024 resolution.
As far as I recall, I have not fiddled with any bits, certainly none at
the system- or device level. Installing user apps like Gimp can't have any
impact, I assume.
What happens:
normal boot: brief purple screen, then a text-mode TTY1 login prompt which
I don't touch because a second later the GUI usually arrives. Except that
now, the GUI doesn't arrive: the TTY1 text-mode screen disappears, there's
an underscore cursor which does not blink, and then nothing more ever
happens. Neither Ctrl-Alt-Del nor Ctrl-Alt-F2 or -F7 work. It seems frozen
and requires a hardware reset.
recovery boot: the GRUB screens take a long time to show; I'm used to the
sub-screens being shown instantly but now there's a delay of maybe 10-20
seconds before the "advanced options" screen is shown. No idea why, nor
whether this is related to the boot problem.
After loading the recovery mode:
several of the recovery options start out with FSCK, and I often get
mountall: fsck /boot [1028] terminated with status 1 on the screen, and
after that nothing more happens. Sometimes I get clean response instead,
but still nothing more happens. Either way, the only keyboard input it
will accept is Ctrl-Alt-Del but this does not reboot the machine right
away! Instead I see a graphical notice the system is running in
low-graphics mode although ironically, this message is displayed at
1600x1200 pixels which my monitor can't handle so it puts its own warning
message in the middle and I can't see the rest of the GUI output. Then,
before I can react, it reboots.
failsafeX has the same problem as the normal boot: it simply freezes after
TTY1.
Hitting Ctrl-Alt-F1 or -F2 or -F7 at various stages doesn't do anything at
all. This makes me think the machine is really frozen, rather than just
working fine but without screen output.
editing GRUB's normal boot entry to remove splash quiet, and to add noapic
nolapic nomodeset doesn't help either. Same situation. Same also when
doing that on the recovery mode's boot entry.
Using the recovery mode to boot the previous kernel version (3.8.0.27)
also has the exact same behavior.
I have already checked this seemingly mandatory post as well as this one
but I couldn't find a solution. This one also looked promising but I can't
get my machine into read/write mode because recovery isn't working and
drop to shell only gives me a read-only system. Setting radeon.modeset=0
also ends in a freeze.
How can I identify the problem, and how can I solve it?
At this point, my computer is completely unusable. I hope I can get this
fixed with your help!
Apologies for any swapped Y and Z characters. I've borrowed my wife's
German laptop to write this post. Oh how I loathe QWERTZ.