First, check to see if it is still visible in other views.

Check Visibility/Graphics. Is the object category (and subcategories) ON?

Check Visibility/Graphics. Is the linestyle set to Override?

Check Visibility/Graphics. Is the detail level set to By View?

Check Visibility/Graphics. Are all the worksets ON?

Check Visibility/Graphics. Are all the filters ON?

Check Visibility/Graphics. If the object is in a linked file check Revit Links.

Check Visibility/Graphics. Check each design option.

Try changing phase and phase filter in View Properties (VP).

Try changing the detail level of the view.

Are all your worksets opened? (it may be hiding in a closed workset).

Check View Range in View Properties (VP).

Check for Plan Regions if in a floor plan view.

The object may have been moved to a new elevation.

Check Crop Region and Far Clip settings in View Properties (VP).

Set your view to wireframe. It may be hiding behind a filled region or other object.

Select the Linework tool (set to By Category) and see if it wasn't hidden with this tool. It would be very benefitial if one could toggle on/off the use of the linework tool in a view.

If the problem is with a family file open the family and check the visibility of the elements.

If you can't find it by now it was probably deleted.