You can change most of your account data or delete your account on the profile page. If you forgot your password, you can request a password reset link from the sign in page (just click on 'Forgot password?' for further instructions).
If you can't sign in it might be a server side issue, waiting a few hours would be the best option. You might encounter any error codes or use your browser's developer tools (console) to identify the error source.
If none of these steps solves the problem, please send an email to [email protected]. Please provide all error codes (if there are any).