Change user password while password is empty [duplicate]

Try running passwd from a terminal. It'll ask you for your old password, and then for your new one.
If it doesn't accept the old one there, run sudo passwd <username> replacing <username> with your username. That will make sure you can change it without the need for your old one, in the hope that it accepts the sudo password.

You will need to boot in to recovery mode (press shift key after BIOS post and keep it pressed until the grub menu is shown) and drop to a root console. There you will be able to change the password for your current user.

Here you can find a very good example of what to do to reset your password.