Histogram missing in ACR. I clicked on something and it went away. Cannot find the way to restore it. Mac OS 26.5.2, ACR 18.4.1, Bridge 16.0.6, Photoshop 27.9.0. Cannot access Adobe help number

Right click in the grey area indicated in the screenshot below, and choose Show Histogram in the context menu.

It is not possible to completely hide or close the main histogram panel at the top of the right-hand panel in Adobe Camera Raw (ACR). The histogram sits permanently at the very top of the primary adjustment panel in the Camera Raw interface. Unlike other collapsible sections or panels in ACR, the histogram area cannot be minimized, closed, or dragged out of view.

No idea what has happened withing your system but I suggest a Camera Raw Preferences reset may be in order.