I can't login with root or any other users. I've tried login in through the computer, SSH and FTP and nothing works. I don't know what caused this because it worked yesterday but suddenly it just stopped.
It's running on Mandrake 10.0, is there any way to repair this from the install CDs or something?

Hi,

Try single user mode. To get there: when presented with the "boot:" prompt (before the kernel is loaded), type "linux single" (it will look like "boot: linux single") and then press enter.
single user mode will not normally prompt you for a password - it dumps you right into a root console (useful for recovery if your system won't boot into any other runlevel, or for when you forget your root password ). If you are asked for the root password (some systems are set to do this, not sure about mandrake tho), I suggest googling for more information. The query "single user mode" + "root password" returned a whole bunch of results.

Good luck.

I have used single user mode before a little, but I am not presented an option to choose which runlevel I want at boot. I do get options though, and I'll try booting it up in safe mode.

It worked! Booting it up in failsafe gave me single user mode. To solve the problem I simply changed the passwords for all the users.

But I don't get it, why did it happen? If this happens again in the future it might give me problems since I won't be able to control it remotely.

WHo knows... Maybe /etc/passwd was corrupted, maybe one of your friends is messing with you, maybe you got H4X0R3D!
Maybe you had caps lock on (j/k)
I would consult /var/log and see what is there....