The battery may not be making a proper connection. Remove and reseat the battery to ensure it is making a proper connection
My headphones will not turn on.
Verify that the battery compartment is free of debris. Use compressed air to blow any dust or debris out of the battery compartment. Use rubbing alcohol and cotton swabs to clean the contacts within the battery compartment
The battery may not be sufficiently charged. Try charging the battery. Fully charging a completely discharged battery takes approximately 2 hours