Shapefile layer no longer editable (toggle edit option greyed out) in QGIS?

I am using QGIS 2.16 / Windows 10.

I have a point shapefile which I have been using for a little while that has always been editable.

Today I was editing the layer, saved my edits and left my computer for a few hours. When I came back I was not able to toggle the editing button on the toolbar and when I right click on the layer it longer has toggle editing as an option.

I have googled the problem every way I can think of and have not been able to find an answer to why the layer is suddenly un-editable or how to fix this.