I am new to GIS but slowly getting the learning curve. I am working with QGIS version 2.18.7. I have been editing several layers (both Raster and vector shape files) but suddenly the 'Toggle Editing' feature is greyed out and therefore I can no longer edit them. Although I have previously been able to do this. I have tried removing Filter but this still did not help.
I have just come across the same problem. Yesterday, I created a new shapefile in my QGIS project and edited it through "Toggle Editing" to add a few polygons. Today, when I reopened my project, the Toggle Editing button was greyed out when selecting this layer in the Layers Panel. The way I solved this problem was by removing the layer from the QGIS project and reloading it again. And voila, the Toggle Editing button became active again when selected the layer in the Layers Panel.