Do you reside abroad and have you lost your Danish citizenship by acquiring another citizenship prior to 1 September 2015?
If you did not apply within the first 5-year transitional scheme, then there is good news for you.
In June of 2021, the Folketing adopted a new 5-year transitional scheme which allows you to regain your Danish citizenship by submitting a declaration. The transitional scheme will enter into force on 1 July 2021 and will be running until 1 July 2026.
You can find the rules on the website of the Ministry of Immigration and Integration, and the Declaration on the Acquisition of Danish Citizenship for former Danish citizens here https://uim.dk/arbejdsomraader/statsborgerskab/tidligere-danske-statsborgere/hvis-man-ikke-opfylder-bopaelsbetingelserne-overgangsordning/.