Recently the labels in Gmail turns to auto-hide model. The labels on the left appear only the cursor hovers on, which is very inconvenient. Please see below how to stop the auto-hide model.
1. Normal Gmail layout: labels such as "Inbox", "Starred" are on the left in the normal Gmail layout.
2. Gmail auto-hide model: labels icons are on the left, and the labels appear only when the cursor hovers on.
3. To stop the auto-hide model and return to the normal Gmail layout, please click the "Main Menu" in the top-left corner.
Click "Main Menu" again and switch back to the auto-hide layout.