What is "Read Render Layer" actually for? I'm in a huge rendering (like over 24 hours into it). And I've accidental click on "Read Render Layer" in the image menu. Then it's like if I lost all of what i was rendering but the computer is still rendering from that point... How can I get the rest back?
Quite frankly never heard of that before.
According to the Blender manual it is used to read cached render buffers from disk, possibly after writting them to save memory
From the Python API manual

bpy.ops.image.read_renderlayers()
Read all the current scene’s render layers from cache, as needed

It is probably used with the Save Buffers option under the Performance tab of the Render panel in the Properties Window which according to the manual:

Saves all render layers and passes to the temp directory on a drive,
and read them back after rendering has finished. This saves memory
usage during rendering, particularly when using many render layers and
passes.

Haven't figured out how to restore image after using it, I'm afraid it is lost, though I don't really fully understand the purpose of this nor how to use it.