My edit toolbar became disabled while I was editing a layer in QGIS. I have closed the window and opened it again to edit, but it is still disabled.
Maybe too obvious an answer, but it's what I needed when googling for this so...
"All editing sessions start by choosing the 'Toggle editing' option"
"By default, QGIS loads layers read-only: This is a safeguard to avoid accidentally editing a layer if there is a slip of the mouse. However, you can choose to edit any layer as long as the data provider supports it, and the underlying data source is writable (i.e. its files are not read-only). "