Hello Nade,
Thank you for posting in Microsoft Community.
I'm sorry to hear that you lost your pen. I just wanted to let you know that Surface Pen does not have a built-in GPS or tracking feature. Hence, it is not possible to track its location using any Microsoft tools and services. However, here is a link you can try to Find and lock a lost Windows device - Microsoft Support.
What I can recommend for now is to retrace your steps and check the places where you may have left your pen. Additionally, you can also try reaching out those locations you previously visited, maybe they have found your pen.
If any case you still cannot locate your Surface Pen, you may need to consider purchasing a replacement.
Kind regards,
Rolando S.