That did happen to me once. If you have a clean site and don't know why it would be banned you can write to them very politely, and if it was due to an error on their part, they may be able to correct it. But it might be a good idea to have someone unbiased review your site and you marketing methods first. Many sites that people complain about here and on other webmaster forums as being nice white hat sites often in reality did something spammy to get themselves banned.
Another reason for a site getting dropped can be technical problems with your site or your host.
[edited by: Jane_Doe at 8:41 pm (utc) on Feb. 17, 2008]
[edited by: Robert_Charlton at 7:35 am (utc) on Feb. 18, 2008]
[edit reason] edited typo [/edit]