Something wierd has happened to the default app icons ( camera, calendar, app store etc). I have 2 copies of each one. The one in the usual place is broken, it won't open the app and if I hold on it it won't wiggle.
Then there is a second copy of each in the top left corner, all on top of each other. That one will launch the app and will wiggle and can be moved.
But if I spread them out, then swipe to move to the second screen of apps I have installed, then come back to the main screen they are all back in the corner.