administration screen won't appear

Hello!
I know this question may sound stupid as I’m feeling so myself right now.

I tried to login to my WP site and that’s fine but I only get the home page of the site. If I click to any link on it, it requests me to login again.
Having said that, I’m not able to go to the Administration screen. I tried to manually typed domain.com/wp-admin and domain.com/wp-admin/index.php but nothing. It takes me back to the login page.

Also, once logged in, shouldn’t I see the links/buttons direct to the Administration screen instead of seeing the home page as any non-admin user?
I’m confused because it worked before and now it doesn’t and I haven’t touched anything since last time.

WP version is 3.4.1 I think as I used the latest available when I set the blog up 2-3 months ago. Also, the blog domain is the root directory.

Please, if anyone can help with this.

Many thanks!