I made a stupid mistake.
I have a shared domain (hosted by hostgator) and I created a subdomain for a blog.
I installed wp to both the domain and the subdomain, both were working fine.
Then I wanted to do some changes and made a stupid mistake: I went to my subdomain's dashboard and at Settings > General I changed the WordPress Address URL and the Site Address URL to the main domain URL.
Now, when I go to http://www.subdomain.domain/wp-login and type in my credentials it takes me to http://www.domain/wp-login and shakes the screen and tells me that there is no such user (as I have different credentials for the main domain). I can login to the main domain's dashboard fine.
I can still see the content on the front end of my subdomain, it's just that I can't get in at the back end.
I am not a very techie guy so please help me to get the login back.