Strange things can happen if /var overfills. This can especially happen if the machine is also a firewall that is logging denied packets.

If you can't get in at all (check ssh, telnet etc), and ctrl-alt-delete doesn't initiate a graceful shutdown,then a hard reboot might be the only solution. Use a boot floppy and check your partitions.

The other possibility is that you've been 0wn3d by some 5cr1pt k1dd13, and your /bin/login has been trojaned. I hope not.