You have have accidently select to hide messages.
Open Microsoft Teams and select the Chat icon from your Teams left hand side App Bar.
Hover to the Microsoft Teams main Search bar.
Then type your client name and hit Enter.
Select the Show hidden chat history hyperlink.
You should see the entire conversation contents for you to read.
You will also note that a new entry showing your client name was added to the list of conversations in the Teams chat app (available at the left hand side of your screen).
Hit the more options next to your client name and select Unhide. From now on the conversation will be visible in your chat list.