Reasons you unable to send and receive messages:

• Check to see whether you have a signal.
• Check to see if your phone is in aeroplane mode. Turn it off if you're using it.
• Ensure that you are using the most recent version of WhatsApp.
• If you have a SIM card, make sure it is properly inserted.
• Make sure you're connected to the internet.
• Reset your APN settings to default in the settings app on your device.
• On my device, there isn't enough room: If you see a message saying that Messages may not be able to send or receive messages until more space is available, ignore it.

Read More: Why is WhatsApp web not generating QR codes on my iPad?