Sometimes people get angry or do not want to associate with others. So, they may block you from their device or app. With WhatsApp, people block others if they do not want to communicate with them. If someone blocks you on WhatsApp, there are limitations that you will have with these people who blocked you.
You can’t call these people and any message you send to them won’t be delivered. There is not a way to determine if this person is online or the last time they used the WhatsApp. I believe that the only way to actually determine if someone has blocked you from them on Whatsapp is to hack into their system, but it is not recommended that you do that.