Failing to show 'Password:' prompt

[Manolis Kiagias]

This morning my server was unresponsive both from ssh and through http. 
The only thing that is different on this system on a 1st of month, is a 
dump job that runs through periodic (on a mostly inactive system).

Visiting the console, I discovered the system was still up and 
displaying login prompt on all vtys, but after typing a username, the 
password prompt would never appear. Keyboard was responsive and I could 
even switch between keyboard layouts (English/Greek). Plugging and and 
unplugging an external USB drive, showed the kernel messages on tty0. 
Still, I could not get it to display a password prompt and had to hard 
reboot it.

I can't find anything in /var/log/messages either - it stops at 5.25 in 
the morning.  And the dump was not performed either (and there are no 
traces about it in the log).

What kind of crash could cause the password prompt to not display? Ideas?