When you mute a chat, it will be identified by a specific icon in the chat overview.

If you see this symbol on any of your chats, it means that you won't receive any push notifications for new messages. You'll only know if there are new messages if you manually open the app and go to your chat overview.

To start receiving push notifications for a muted chat again, you can click on the arrow located to the right of the chat name. After that, simply select "Turn Notifications On":

Once you've done this, you'll start receiving push notifications once more whenever there are new messages in the chat.