Hey @Anusha Racheal from the support engineer team here 

There are a few reasons this could disappear! The most common are, your Messenger Settings here (make sure you check for users and visitors):

And another common reason would be volume control. When a customer uses the Messenger, all your assignment rules work in the background to predict which team inbox the conversation is more likely to get assigned to. If all teammates who belong to the predicted team inbox have reached their assignment limit or are set to away, the customer won’t see the “New conversation” button in the Messenger. 

Hi @Racheal 

We are having a similar issue.
The "Send us a message" option is missing but also the layout of messenger looks different?
Any ideas?

We tried your above suggestions but they don’t seem to work?
This issue is only happening on the web app, the mobile app is working fine. 

Thanks!

Update: We sorted it!