Admin User Name and Password problem on subdomain

I have 2 questions.

I have a domain "www.homedecoronline.net" and a subdomain "homedecoronline.net/dev"

I am learing how to build a site so I missed up after installing the Zen cart to the main domain" Some files are deleted, the template is deleted" So I want to know how can I reinstall the Zen Cart on my main domain. I installed it using the Fantastico Deluxe.

Then I installed the zen cart to the subdomain and Iam trying a little by little editing the Defualt template just to get a feel of it before I install one of the Zen templates such as the Apple Zen or Cherry.

Now the problem on the Subdomain is I cant get into to the admin"www.homedecoronline.net/dev/admin"

The page where you can insert the username and password comes on and I input the username/password and within second the fields clear and no error comes on. If Insert a wrong username/password. It says that wrong name or password.

What can be wrong?

P.S. I can login in the admin panel for the main domain.

I fixed the first problem by ftping the default template to the /include/templates and now Igot the default template back.

But stil I can't login into the admin panel on the subdomain