I'm using Blender 2.8. I split my viewport but accidentally moved the 3D manipulator to the right side- I don't know how I moved it nor how to move it back to the left side. The ride side is in rendered view. When I try to use the measure tool, manipulator and other tools on the left side, I get an alert that says 'gizmo hidden in this view'.
It appears that you've hidden the gizmos in the left viewport, which is why the Move gizmo is invisible and the Measure tool gives you an error message. Enable them by clicking the Show Gizmos icon shown below.
Ensure that the following options are enabled in Viewport Gizmos.
The exact error message is "Gizmos hidden in this view" which is printed by
view3d_gizmo_ruler.c) when the Measure tool is used and
v3d->gizmo_flag has the
V3D_GIZMO_HIDE_TOOL flags set.
V3D_GIZMO_HIDE is set when Show Gizmos is disabled and
V3D_GIZMO_HIDE_TOOL when Active Tools in the Viewport Gizmos options is deactivated.