Your first step would probably be to find out what the SIS says about you. Maybe someone else used your identity, or there is a mismatch.
According to the European Commission's website:
What is the procedure for requesting access to personal data in the SIS?
If you believe your personal information has been misused, needs to be corrected or deleted, you can request access to and rectification of your data. If you are a third-country national you can address your request to the consulate of any Member State.
The European Data Protection Supervisor's website also says:
The SIS II Regulation provides persons with a right of access and a right to correction of inaccurate data and deletion of unlawfully stored data.
If you want to know which personal data are processed in the SIS II or if you want to correct or delete your data, because they were wrongfully entered in the SIS II, you can make a relevant request in any Schengen country, by contacting the competent authority. If you are currently outside the Schengen Area you can also contact the consulate of a Schengen country in the country in which you currently live. You will be informed about the follow-up of your request within three months at the latest.
The Group has adopted a Guide for exercising the right of access which provides you with detailed information on your rights under the SIS II legal framework. The Guide also lists all the competent authorities in the Member States and contains two model letters, one for the right of access and one for the right of correction or deletion.