Maybe the password had an expiration date or the user account is locked for some reason. As root, you can go in Yast --> Security and Users --> User Management, then select the user and click on edit. Look around if there is something strange: in particular look at the "Disable User Login" box and be sure it is unchecked.
Otherwise, as root try to reset password for the user, as sophkos already suggested.
Originally Posted by hellknight87
there is important data lying in my home partition which i want to access. I don't want to reformat and install Linux again.
You will not lose any data, as long as you have access as root. But always consider to do a backup of your valuable data, somewhere (on an external device).