(Moto X Play) why can't I use my centre button?

[Android Central Question]

I have been receiving notifications on my moto X Play urging me to remove apps etc as I was reaching the max storage.

So I cleared some space: removed an app and put most of my photos in archive (Google photos) or just deleted them.
I now have 8.52GB in storage out of 10.89GB of storage.

But now I can't use the centre "home" button or the minimizer button.

Why is this?

Please could anyone give me any tips on how to resolve this?