Failing the above...
...it may need a PS/2 keyboard. Some BIOS doesn't like USB. Unless you're using a PS/2, in which case you may need a USB. Or something. Regardless, you need to hit it, and probably hold it, before GRUB grabs everything.
EDIT: And try holding 'Del' instead. Or F10.
ANOTHER EDIT: You can press 'Pause' on some loaders to pause the screen text. It may give you some clue.
(ThomsonsPier consumes, Fri 16 Oct 2009, 14:53, 1 reply, 16 years ago)

wireless, yet PS/2
but it's always worked before, it goes "detecting IDE drives" whilst giving me the option to press Del to enter setup.

I don't know why I said it was F2, but either way f2 doesn't work either.
( sandettie light vessel automatic New Twitter - @bollocksreally, Fri 16 Oct 2009, 14:56, Reply)

Try a wired keyboard.
If the OS hasn't successfully loaded and configured yet, it might not like the wireless keyboard. Plug a standard one in until you can make sure it's happy.
(ThomsonsPier consumes, Fri 16 Oct 2009, 14:58, Reply)

Surely
all this happens before the oS is even looked at.
( sandettie light vessel automatic New Twitter - @bollocksreally, Fri 16 Oct 2009, 15:01, Reply)

You'd think so, wouldn't you?
I've had a keyboard not recognised just because I plugged it into a different USB socket. After the OS loaded and I rebooted, it worked fine.
(ThomsonsPier consumes, Fri 16 Oct 2009, 15:06, Reply)

Often a hardware change will force the machine to prompt to enter setup
If you've got enough memory in there pull a stick out temporarily and it should error on bootup and offer to enter setup.
(bill, Fri 16 Oct 2009, 15:35, Reply)