Snapchat are clamping down on the use of third party apps (as found on Windows phones for example) as they feel that these are not secure. If they find that you have been using this method they will send you an email requesting that you reset your password and use the correct app. If you ignore the email they will lock your account.
Alternatively, perhaps you have broken one of their rules. For a list of these just go to https://www.snapchat.com/terms and look under "Prohibited activities". Sometimes you might do something that you didn't realise was a violation of their rules, if that is the case then hopefully you can explain this to them.
The best way to find out why your Snapchat account has been locked is to contact the team at Snapchat and ask them. I doubt they would have locked your account without first notifying you that this would happen, but maybe that email went missing.
You can log into support at https://snapchat.com/unlock after which they will unlock your account and send you a new password.
If you don't get a reply, try tweeting about your problem to @snapchatsupport Speaking publicly about your issue is more likely to get a favourable and swift response.