Short answer
The couple should first try to understand why the change has occurred. Stress, emotional hurt, health issues, medication, exhaustion, relationship conflict and other personal circumstances can affect interest.
Islamic Guidance
What Islam teaches
Marriage involves mutual rights as well as kindness and mercy. Neither spouse should respond to a sensitive issue through humiliation, coercion or cruelty.
Practical Guidance
What you can consider
Have a private and non-accusatory conversation. If the issue persists, consider appropriate medical or marital counselling to identify the underlying cause.
Sources
Qur'an 4:19; Qur'an 30:21.