The most common reason why you can't connect to WhatsApp is because your phone is temporarily not connected to the Internet.
To ensure you have a working Internet connection, go through the following steps:
If you can't connect to the Internet through your phone's data network, try one or more Wi-Fi networks. If you can open webpages and run other applications that connect to the Internet on your phone, but WhatsApp won't connect, learn more about connection issues on: Android | iPhone
If you can connect to other applications on your data network, but not WhatsApp, it could be that your mobile provider has configured your wireless access points to block certain chat applications. Try this if WhatsApp still doesn't work on data only: