IMPORTANT: See Reasons Why WHATSAPP Is Banning Users Recently - Don't Fall A Victim

This might be the reason ,
source : Why am I seeing the message “You’re temporarily banned from WhatsApp"?
If we suspect that your account activity constitutes an abuse of our system, your account will be temporarily banned from using WhatsApp. You will receive a warning message in the app that contains the duration of this temporary ban.
During the temporary ban you will not be able to access any part of your WhatsApp account. You will be able to use your account again after the countdown timer runs out.

Activity that can lead to this temporary ban includes:

• Sending too many messages to users who do not list you in their address books. Make sure that you ask all of your WhatsApp contacts to add your current number to their address books.
• Being blocked by too many people in a short period of time. Make sure you only communicate with people who want to receive messages from you.
• Creating too many groups that contain users who do not list you in their address books. Make sure that you ask all of your WhatsApp contacts to add your current number to their address books.
• Sending the same message to too many people. Use Broadcast Lists to send the same message to your contacts.
• You may have violated our Terms of Service.Sending third party advertisements or publishing any other kind of commercial listings..
• By Submitting some content status or display pic which is offensive or hurtful or it may contain nudity
• Using third party alternatives like Whatsapp plus and OGwhatsapp which act as automated bots and can access all files and data transferring through whatsapp, Can’t blame whatsapp because these modded apps makes threat to Whatsapp users privacy policy.

If you are using third party softwares like Whats app plus 

• the first step you must do is to uninstall the modded version you installed in your device.
• Here is the link to the support team of whatsapp [support@whatsapp.com] and you can mention that your account had blocked by mistake.
• They will check in detail and revoke the action if it is made by mistake. While you are in a blocked mode your friends can’t see you in their contact list and also they can’t message you.
• There is nothing more you can do in this situation.