Try these troubleshooting tips:
1. Put both earbuds back in the case, close the lid, and try again.
2. Ensure that both the case and earbuds are charged.
3. Clean the inner side of the earbuds and ensure there is no dust on the wear detect sensor. Use a soft, lint-free cloth, such as a lens cloth or microfiber towel. Then, put them back in the case, close the lid, and try again.
If the issue persists, perform a factory reset.