The chat's gone.
Hi @Rita1986,
can you check if you have perhaps unpinned the chat with the mouse right click by accident?
If not, I suggest you to delete the cache on your Microsoft Teams app:
If Teams is still running, right-click the Teams icon on the taskbar, and then select Quit.
Open the Run dialog box by pressing the Windows logo key + R
In the Run dialog box, enter the following path, and then select OK.
%userprofile%\appdata\local\Packages\MSTeams_8wekyb3d8bbwe\LocalCache\Microsoft\MSTeams
Delete all files and folders in the directory
Restart Teams.
Clear Teams cache - Microsoft Teams | Microsoft Learn
If that doesn't help you can try to sign-out from Microsoft Teams and then sign in.
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Kindest regards,
Leon Pavesic
(LinkedIn)