If the internal tip of your Apple Pencil Pro is missing, it would explain why it is no longer responsive. The internal tip houses the contact sensor required for accurate touch input and pressure sensitivity.
Look closely inside the tip area where the internal component should be. Check if the internal sensor (metallic or spring-like part) is loose, dislodged, or completely missing.
If the internal tip is completely gone or irreparable, you will need to contact Apple Support for further assistance with this matter. To do so, simply select the "Support" link on the lower left-hand side of this forum's web page.