Apple Pencil stopped working after changing the tip

First, ensure that the replaceable plastic pencil tip is screwed firmly into the body of the Apple Pencil; even a slightly loosened tip will cause issues. When correctly positioned, there should be a tiny gap (of about the thickness of a sheet of paper) between the tip and the body of the Pencil.

If you have damaged the fragile metal transducer, located beneath the replaceable plastic tip, then the Pencil may have been irreparably damaged. In this situation your Pencil will require total replacement.

Assuming that the transducer is not distorted or otherwise damaged, ensure that the Pencil is properly paired with the iPad. This support page may be helpful:

Connect Apple Pencil with your iPad - Apple Support

If you continue to experience difficulties, here is one of the better troubleshooting guides:

https://appletoolbox.com/apple-pencil-not-working-heres-our-troubleshooting-guide/

Should difficulties persist, you would be well advised to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess both your iPad and Pencil:

Genius Bar Reservation and Apple Support Options - Apple

OK. Is the Pencil registering when you place it on the charger on the iPad directly? When you go to Settings > Bluetooth is the Pencil showing as connected?

OK. Perhaps the new tip is defective. Usually, when you buy authentic tips from Apple, they come in a pack of 4. Did you try a different tip?

Did you buy an aftermarket tip or an authentic Apple tip?