You did something to trigger a security check.
If you can't remember the answers to the security questions, then go to Express Lane and select 'iTunes' from the list of 'products' in the middle of the screen.
Then select 'iTunes Store', and on the next screen select 'Account Management'
Next choose 'iTunes Store Account Security' and fill in that you'd like your security questions/answers reset.
You should get an email reply within about 24 hours (and check your Spam folder as well as your Inbox).
More info, like the rescue email address
http://support.apple.com/kb/index?page=search&src=support_site.results.disambigu ation&q=Apple%20ID%20has%20been%20disabled%20for%20security%20reasons