1st try to find the possible reasons behind of this issue.....
Most likely, someone changed the root password. which could mean that the box has been hacked / the passwd or shadow file got corrupted or deleted somehow / problem in /etc/nsswitch.conf or ur PAM configuration.
However u can try this:
Reboot Linux, at the grub prompt, edit the kernel line and add the line as init=/bin/bash. When it boots, u will get a bash prompt, and the root fs will be mounted read-only. U will need to remount it RW, then change the root password and run sync, again remount / (root) as ro, and pass the command likeL reboot -f now
OR u can try this way also:
1. boot in rescue mode
2. mount /dev/sdaX /mnt (here X means type the number and assuming sda is
ur HDD id)
3. mount --bind /dev /mnt/dev
4. chroot /mnt
5. passwd root