Look in the Layers palette for the word Lock just below the blending mode menu. One of the lock options represented by the 4 icons next to Lock is activated. Click on the darkened one.
On the layers palette, at the top of the list of layers is a heading called "Lock" with various symbols after it. One of them will be highlighted. Click on it to un-highlight it. Then the layer will be unlocked.
See also the Photoshop - Help section under "Locking layers"
You could check your document setup (bit size)
and if it is set at 32 or 64 bit, make it 8 bit and this will unlock the doc.
None of the proposed ways works for me.
The little lock icons do not repond and the word Lock: is grayed.
Could it be because it is a .pdf document?
Doesn't work after I Save As .jpg either.
Nor if I first save it as .psd.
if it's the backgrond layer that is always locked. double click on the layer and rename it that should unlock it
Yes, thanks, that unlocks it.
But then comes the next problem:
On top of that layer I have an adjustment layer (Curves), looks fine, but when I merge it down or flatten the image
the effect of the adjustment layer disappears.
I've been merging adjustment layers down like this for many years, all ok.
So why not now?