You need to contact steinberg support. Since your key is lost, your license is also lost, and it´s Steinberg who decide, if they give you a new license for free, or if you have to buy it yourself.
https://www.steinberg.net/en/support/knowledgebase_new/show_details/kb_show/usb-elicensersteinberg-key-details-tips-and-troubleshooting/kb_back/2020.html