New installation – unable to access wp-admin

I have set up about 20 wp sites in Hostgator using Fantastico. However the two sites I set up this week both have the same problem. When I try to go to the login page for wp-admin (www.mysite.com/wp-admin) I stay on the homepage.

If I use http://www.mysite.com/wp-login.php, I can get to the login page and can then put in my login details but still get taken back to the homepage. No error messages.

For the first of the two problem sites I contacted Hostgator and they were able to sort it. Unfortunately, I did not ask them what they did! Now I have the same problem today and they are unable to help me as they say it must be a script problem.

Does anyone have any ideas? Thanks