Password at startup

My iMac Desktop 20" 10.7.5 Lion had password on startup . clicked on users,them changed password to blank (no Password) so that no password request on startup. Next startup, Password was requested again. Put in previously used password but was rejected. Now can't start mac as doesn't accept old password. This solution to fix was on Apple's forum site but has resulted in no access to start .
Would greatly appreciate any assistance as to how I can resolve the password problem.

Tried not putting anything in the password field?
If you get logged in, go to Users and Groups in System Preferences, click the lock to unlock.
Put in a new password. Now click on Login Options and set Automatic Login in the dropdown.

Password At start up

Thanks so much ,Cradom. Fixed the problem. Many thanks