Conversation sessions are limited to a 24 hour timespan.
After 24 hours of inactive chat, WhatsApp requires businesses to use the template feature to initiate conversations with customers. These templates are approved ahead of time to make sure they’re transactional in nature and are not cold messages or promotional/advertisement messages. If you tap the template button, you'll see a list of messages that you're able to send to the customer. Tap the message to select it, then press the send button.