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:

1. Make sure you have a data plan on your carrier's network or a Wi-Fi connection with Internet access.
2. Make sure that the signal strength of the Wi-Fi or data network is good and the phone is connected to the network.
3. Open a webpage in your phone browser to see if it loads. If it does, your phone is connected to the Internet.
4. Reboot your phone.

If you still can't connect

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:

1. Contact your mobile provider and ask them about different access point settings to run chat apps.
2. They will guide you through a process in which you will manually configure your data network and web settings.

Learn more about connection on: Android | iPhone