Log in iMac problem, Log in iMac problem

Hello..

I am having a problem signing in on my iMac (Lion)
Last night i turned my computer off as normal, no problems what so ever, but today i turned it on and it asked for a log in password which is did NOT set, and i don't usually need a password to log in to, i have tried every password i have had in the last 2 years and non of which work... i have tired my Apple account ID, and that does not work either, i know the password for my Apple ID is correct due to signing in to my Apple account and infact sending this message..

I have absolutely know idea, to how my iMac as automatically set a log in password for me?

Any ideas how to solve this problem would be greatly appreciated

I have also had the harddrive replaced in February, due to a fault old one..