I overstayed by 12 days in Sweden. The last time I applied for a short-term Schengen visa it was denied because of the overstay. How can I find my personal data in the Schengen information system about my penalty?
As was noted by @GayotFow, you can make an Subject Access Request to Sweden and/or to the country which refused your visa application (should it be other than Sweden).
The Schengen Information System Guide for Exercising the Right of Access provides the contact details for Sweden, at page 62 (as well as the other Member States):
Requests for access must be made to the National Police Board (Rikspolisstyrelsen), which is the authority responsible for the Swedish unit of the Schengen Information System.
S - 102 26 Stockholm
Tel.: ++46 (0)8-401 90 00
Fax: ++46 (0)8-401 99 90
Requests must be made in writing to the National Police Board and signed personally by the applicant. In general, a request for access must be answered within one month. Applicants are entitled to free access to information once every calendar year.