I have some items in Xcode storyboard suddenly become invisible as shown in the picture. I don't know what happened and how to make it back to normal.
Not only pickerView in the picture meet such problem, but also buttons in other Controller View meet the same problem.
Any help or link is appreciate.
It's because when you added the Picker View you were in a specific size class, and now you are not in that size class. You have made the Picker View a conditional view (conditional upon the view controller being in that size class at a given moment).
It's not invisible but absent from the view controller's view in the current size class, because it's a different size class. Xcode is merely doing what you asked it to do, i.e. include this view only when in the other size class.
If appears with gray color it means that you are using "size classes" and you have selected a class in which that pickerview is "uninstalled".
To install the view for your current size just go to interface builder and mark the option "installed"
For example, in the image attached, im just installing that view for Regular width and regular height == just for iPads in Portrait and Landscape
